With Christmas just past and more Commander products on the horizon, now seems like an ideal time to discuss the process of upgrading the preconstructed deck you have just purchased or been given.  Here I will outline a general procedure for doing so and use a poor, innocent Primal Genesis deck as our victim (example).  Deck lists for the original Primal Genesis and the upgraded version are at the end.

Step 1:  Pick your Commander

Magic precons usually have a couple of generals included.  Primal Genesis has three:  Ghired, Conclave Exile, Atla Palani, Nest Tender, and Marisi, Breaker of the Coil.  The problem is the generals included often have different strategies suited to them.  Ghired has the feel of a token-based strategy.  Atla wants a deck filled with big, dumb, little kid creatures to get with your eggs and Marisi is ... included to be a third general.  The main choice here is between Ghired and Atla.  This will determine a direction for almost all of the rest of the upgrades.  Looking at the two of them I feel that it is obviously time to break some eggs.

Out: Ghired, Conclave Exile, Marisi, Breaker of the Coil.

Step 2:  Look at the Mana

Precon mana is awful.  The colors are usually fine but there tend to be too many lands coming into play tapped or with simple, cheap replacements that would work better.  Another issue is lands are often tossed in a precon to satisfy the need for a reprint, but it's only mediocre for deck.  Upgrades here have an enormous impact on the playability and enjoyment of your deck.  

As you work your way through the other steps, you will want to revisit this step repeatedly.  Looking at Primal Genesis, it has 12 lands that come into play tapped (here I'm counting Terramorphic Expanse and Evolving Wilds).  Having almost a third of your lands coming into play tapped is too slow.  Many games this will put you a turn behind everyone mana development-wise.  One of the first upgrades should be to cut almost all of the come into play tapped lands and make better choices.  Terramorphic Expanse and Evolving Wilds are fine keeps as they are color fixing for your mana.  Krosan Verge can stay as well.  It grabs you two lands and can get Cinder Glade or Temple Garden if you make that upgrade.

Exotic Orchard feels like it is included more for reprint value than for the deck.  Gargoyle Castle is sub-optimal even in a pure token strategy.  With lots of eggs, it's terrible.  Myriad Landscape is a nice land, but since it only grabs two of the same basic land, it doesn't have as much of a place in a three color deck.  I have no idea why Rogue's Passage is in the deck.  My guess is for Marisi, Breaker of the Coil, but he's gone, so no need to keep it.  Cinder Glade and Sungrass Prairie aren't bad land choices, especially when your upgrade adds Canopy Vista and Mossfire Valley.  I have never been a fan of the Boros Garrison Karoo lands (unless you have some strategy to specifically take advantage of them), so these get cut.  Even though Jungle Shrine also comes into play tapped it can stay because getting all three colors of mana we need is probably worth accepting our fourth comes-into-play-tapped land.

Out:  Exotic Orchard, Gargoyle Castle, Blossoming Sands, Boros Garrison, Graypelt Refuge, Gruul Turf, Kazandu Refuge, Myriad Landscape, Rogue's Passage, Rugged Highlands, Selesnya Sanctuary.
In: Mossfire Valley, Canopy Vista, Temple Garden, Stomping Ground, Sacred Foundry.

Step 3:  Look at your strategy (remove what doesn't fit)

For this step, you need to look at what parts of the precon support the strategy you have opted for.  See what plays well with your general and what doesn't.  With Atla, the plan is to make eggs to sacrifice to get big creatures into play cheaply (and hopefully at instant speed).  This means that most of the creatures in the precon are on their way out.  You want to get the most out of every egg you crack.  

A creature either needs to do something exciting or be huge (or hopefully both).  Feldon of the Third Path gets to stay.  He works well with the big creature plan since he can pull them out of the graveyard.  It may sound a little strange but Sakura-Tribe Elder gets the axe.  He isn't amazing enough to get out of an egg and we have other non-creature  mana acceleration options that won't dilute our egg cracking chances.  Desolation Twin goes because he makes his copy on cast not upon coming into play.  That will also keep us away from most of the other Eldrazi.  Roc Egg is one of four actual eggs legal for us to play, along with Rukh Egg, Dragon Egg and Summoner's Egg (sadly no Ludevic's Test Subject for us - it was errata'd to Lizard Egg because of reasons?).  This feels like it falls in the area of being too "cute" but the Roc and Dragon eggs are both pretty inexpensive to cast and Summoner's Egg has the potential to get an expensive creature stuck in our hand into play.  Giant Adephage might seem like it should stay based on the size, but it really just ends up being a plain vanilla 7/7 creature if you aren't doing any token doubling.  Rampaging Baloths and Thragtusk can stay for now but probably not longer term.  

The rest of the creatures in the precon get replaced.  Maybe with the exception of Heart-Piercer Manticore.  This one would be one of my last cuts and it might get to stay if the deck ends up having enough large creatures to sac for extra damage or effect.

Out:  Doomed Artisan, Tectonic Hellion, Ohran Frostfang, Selesnya Eulogist, Tahngarth, First Mate, Angel of Sanctions, Wingmate Roc, Dragonmaster Outcast, Giant Adephage, Soul of Zendikar, Trostani, Selesnya's Voice, Desolation Twin, Flamerush Rider, Emmara Tandris, Wayfaring Temple, Cliffside Rescuer, Voice of Many, Scaretiller, Garruk's Packleader,  Sakura-Tribe Elder, Vitu-Ghazi Guildmage.
In:  Nothing yet.

Step 4:  Enhance your Strategy

What can you add that contributes to your chosen strategy?  In the case of Primal Genesis it's time for the monsters.  I feel like the first creature to add kind of has to be Nesting Dragon, which checks all the boxes.  It's a little kid creature and it makes eggs.  Since we are talking about the egg theme, I think a few good changelings are in order.  Chameleon Colossus, Taurean Mauler, Mirror Entity and Universal Automaton aren't bad eggs.  I'd like to include Changeling Hero and Changeling Titan but if you egg into them as your only creature, that makes that one a miss.  Getting two eggs from Irregular Cohort seems like it also might be a good choice.  Time for the Summoner's Egg and Dragon Egg as well.

After the cute it's on to the big monsters.  Worldspine Wurm seems like about the right size.  Throw in Akroma, Angel of Wrath and her friend Zetalpa, Primal Dawn.  Ilharg, the Raze-Boar works as a big monster and will help us use any expensive creatures trapped in hand.  Dragonlord Dromoka may seem like a little bit of an odd choice, but he is big enough to be worth it and very disruptive to your opponents while also being quite castable from your hand.  Avacyn, Angel of Hope and Zacama, Primal Calamity go in too.  Although you don't get the untap from Zacama if you hatch it, you still get a 9/9 out of the deal.  Drakuseth, Maw of Flames feels right for the deck and Elesh Norn, Grand Cenobite can help clear out pesky small creatures (and help avoid some of the combos based around them from popping up).  Moldgraf Monstrosity and Stalking Vengeance are both big creatures with interesting tricks.  Eternal Witness, Acidic Slime and Woodfall Primus help round things out with a little utility.

Out:  Nothing.
In:  Nesting Dragon, Chameleon Colossus, Taurean Mauler, Mirror Entity, Universal Automaton, Irregular Cohort, Summoner's Egg, Dragon Egg, Worldspine Wurm, Akroma, Angel of Wrath, Zetalpa, Primal Dawn, Ilharg, the Raze-Boar, Dragonlord Dromoka, Avacyn, Angel of Hope, Zacama, Primal Calamity, Drakuseth, Maw of Flames, Elesh Norn, Grand Cenobite, Moldgraf Monstrosity, Stalking Vengeance, Eternal Witness, Acidic Slime, Woodfall Primus.

Step 4:  Iterate Steps 2 and 3

You will want to repeat steps 2 and 3 looking at the rest of the deck.  It may have become obvious by now looking at Primal Genesis that the deck was more token based than egg based.  Ghired's Belligerence, Full Flowering and Hour of Reckoning further illustrate this.  Lots of populate and tokens.  This isn't bad but could have been far better if we went in a different direction with Ghired, Conclave Exile as the general.  We could have leaned into the token generation by adding Parallel Lives, Doubling Season and Anointed Procession.  Since there are only eggs being made, the populate cards can be replaced with either better versions or different cards altogether.  To help with mana, these can come out for Kodama's Reach, Explosive Vegetation and Skyshroud Claim (thanks to the mana updates earlier).

I have to admit to being a little sad about not being able to cast Hate Mirage with a couple of token doublers out.  The card can stay regardless, but I feel it could really shine in another deck.  Cultivate, Harmonize and Farseek are fine.  Same with Shamanic Revelation.  It's only ok card draw, but for Naya (Green/Red/White) you can do worse.  Explore is replaced with Fertile Ground.  I would rather have definite any color mana instead of maybe an extra land drop (if you have the land) and a card draw.  Make sure to put the Fertile Ground on a basic land if you can to make it a little harder to get 2-for-1'ed.  I would prefer more versatility than Phyrexian Rebirth offers so this gets replaced by Austere Command.

Out:  Ghired's Belligerence, Full Flowering, Hour of Reckoning, Explore, Phyrexian Rebirth.
In:  Kodama's Reach, Explosive Vegetation, Skyshroud Claim, Fertile Ground, Austere Command.

Looking at the instants in the deck, Momentous Fall gets better with eggs due to our big creatures.  Beast Within is almost universally useful being able to hit any permanent.  Fresh Meat gets swapped out for a second one with Generous Gift.  Slice in Twain is a trap.  I've tried to play this card multiple times and every time it has felt clunky and bad.  It seems like it should be good being a cantrip, but it really isn't.  Naya Charm is ok but awfully situational.  If I am going with this kind of charm, I'd rather have Rith's Charm.  Either way, neither makes the cut.  Same with the other token/populate instants.  Better options exist after taking a look at the rest of the deck.

Out:  Fresh Meat, Naya Charm, Second Harvest, Rootborn Defenses, Trostani's Judgment, Druid's Deliverance, Slice in Twain, Sundering Growth.
In:  Generous Gift.

Artifact-wise, Sol Ring and Lightning Greaves are keepers.  Idol of Oblivion can go.  Soul Foundry as well because we don't want to pay the absurd costs for our big creatures that might get imprinted.  Mimic Vat is a definite keep.  Even without any token manipulation, this card can be amazingly disruptive to a game and other people's strategies.  It kills a lot of graveyard combos if you imprint a critical creature they need to cycle and keep it on the vat.  All the enchantments can go.  Once again, we can do better for our strategy.  Garruk, Primal Hunter can stay because he is card draw.

Out:  Idol of Oblivion, Soul Foundry, Commander's Insignia, Song of the Worldsoul, Growing Ranks, Intangible Virtue, Colossal Majesty, Elemental Bond.
In:  Nothing Yet.

Now that we've figured out everything that doesn't fit our strategy, time to fill in with things that do.  Since we need eggs to die, we will want some sacrifice outlets.  Goblin Bombardment is an easy choice.  Same with Skullclamp.  It isn't exactly sacrifice, but it does kill eggs.  Martyr's Cause and Thermopod are both interesting sacrifice options to try out.  Ashnod's Altar and Phyrexian Altar round things out and maybe provide a little mana.  Solemn Simulacrum is good enough to waste an egg on so he gets included to help further fix our mana.

Because the eggs reveal cards from the top of the deck putting creatures on top will give us some extra control of things. Worldly Tutor and Scroll Rack help with this.  With the sacrificing for mana options we've added, I feel like an Avenger of Zendikar is a good include now.  It was good before, but it seems like almost every green EDH deck is packing one.  I was trying for extra variety, but you can't argue with the power of the card in Commander.  Altar of Bone seems like a fun choice.  This will probably come out later, but I want to give it a shot.  High Market is another option for saccing eggs, and it works toward fixing our land deficit from earlier.   Samut, Voice of Dissent gives our big creatures haste and untaps Atla for an extra egg if we have nothing more exciting to do.  Arcane Signet lets us pick up a little extra mana and mana fixing.  Sylvan Library gives us a limited way to set up egg hatchings.  Etali, Primal Storm helps us take extra advantage of being able to set up the top of our deck.  Kiki-Jiki, Mirror Breaker is an idea, but I think there are too many legendary creatures in the deck to use him to his fullest advantage.  He is also much more fun with token doublers.

Triumph of the Hordes and Overwhelming Stampede both make pretty solid finishers if you need to push through damage.  The choice here is largely determined by how your playgroup feels about poison.  To be safe, I will go with the Stampede.  Aura Shards can go a long way to solving random problems that might pop up.  At this point, we can finish things up with some basic lands and a Wayfarer's Bauble and head to the card shop.

In:  Goblin Bombardment, Skullclamp, Martyr's Cause, Thermopod, Ashnod's Altar, Phyrexian Altar, Solemn Simulacrum, Worldly Tutor, Scroll Rack, Avenger of Zendikar, Altar of Bone, High Market, Samut, Voice of Dissent, Arcane Signet, Sylvan Library, Etali, Primal Storm, Overwhelming Stampede, Aura Shards, Wayfarer's Bauble, + adjust the basic lands.
Out: Nothing.

Wow.  That seemed like a lot.  Odds are you won't make the modifications to a precon like I did above all at once.  Building and updating an EDH deck is an ongoing process.  You will go through the steps over and over again during the life of the deck.  Sometimes you may even go all the way back to step one and swap generals.  

I hope this article will help with upgrading and modifying the next precon you run across or maybe inspire you to pick up Primal Genesis and go the other way, populating as you go.

Primal Genesis (Original Precon)

1 Garruk, Primal Hunter
1 Ghired, Conclave Exile
1 Atla Palani, Nest Tender
1 Marisi, Breaker of the Coil
1 Doomed Artisan
1 Tectonic Hellion
1 Ohran Frostfang
1 Selesnya Eulogist
1 Tahngarth, First Mate
1 Angel of Sanctions
1 Wingmate Roc
1 Dragonmaster Outcast
1 Feldon of the Third Path
1 Giant Adephage
1 Soul of Zendikar
1 Trostani, Selesnya's Voice
1 Desolation Twin
1 Flamerush Rider
1 Heart-Piercer Manticore
1 Rampaging Baloths
1 Thragtusk
1 Emmara Tandris
1 Wayfaring Temple
1 Cliffside Rescuer
1 Voice of Many
1 Scaretiller
1 Roc Egg
1 Garruk's Packleader
1 Sakura-Tribe Elder
1 Vitu-Ghazi Guildmage
1 Ghired's Belligerence
1 Full Flowering
1 Hour of Reckoning
1 Phyrexian Rebirth
1 Shamanic Revelation
1 Hate Mirage
1 Cultivate
1 Explore
1 Farseek
1 Harmonize
1 Fresh Meat
1 Momentous Fall
1 Second Harvest
1 Rootborn Defenses
1 Trostani's Judgment
1 Beast Within
1 Druid's Deliverance
1 Slice in Twain
1 Naya Charm
1 Sundering Growth
1 Idol of Oblivion
1 Mimic Vat
1 Soul Foundry
1 Lightning Greaves
1 Sol Ring
1 Commander's Insignia
1 Song of the Worldsoul
1 Growing Ranks
1 Intangible Virtue
1 Colossal Majesty
1 Elemental Bond
1 Cinder Glade
1 Exotic Orchard
1 Gargoyle Castle
1 Sungrass Prairie
7 Plains
4 Mountain
8 Forest
1 Ash Barrens
1 Blossoming Sands
1 Boros Garrison
1 Command Tower
1 Evolving Wilds
1 Graypelt Refuge
1 Gruul Turf
1 Jungle Shrine
1 Kazandu Refuge
1 Krosan Verge
1 Myriad Landscape
1 Naya Panorama
1 Rogue's Passage
1 Rugged Highlands
1 Selesnya Sanctuary
1 Terramorphic Expanse

Primal Genesis (Updated Version)

1 Acidic Slime
1 Akroma, Angel of Wrath
1 Altar of Bone
1 Arcane Signet
1 Ash Barrens
1 Ashnod's Altar
1 Atla Palani, Nest Tender 
1 Aura Shards
1 Austere Command
1 Avenger of Zendikar
1 Beast Within
1 Canopy Vista
1 Chameleon Colossus
1 Cinder Glade
1 Command Tower
1 Cultivate
1 Dragon Egg
1 Dragonlord Dromoka
1 Drakuseth, Maw of Flames
1 Elesh Norn, Grand Cenobite
1 Etali, Primal Storm
1 Eternal Witness
1 Evolving Wilds
1 Explosive Vegetation
1 Farseek
1 Feldon of the Third Path
1 Fertile Ground
10 Forest
1 Garruk, Primal Hunter
1 Generous Gift
1 Goblin Bombardment
1 Harmonize
1 Hate Mirage
1 Heart-Piercer Manticore
1 High Market
1 Ilharg, the Raze-Boar
1 Irregular Cohort
1 Jungle Shrine
1 Kodama's Reach
1 Krosan Verge
1 Lightning Greaves
1 Martyr's Cause
1 Mimic Vat
1 Mirror Entity
1 Moldgraf Monstrosity
1 Momentous Fall
1 Mossfire Valley
6 Mountain
1 Naya Panorama
1 Nesting Dragon
1 Overwhelming Stampede
1 Phyrexian Altar
8 Plains
1 Rampaging Baloths
1 Roc Egg
1 Sacred Foundry
1 Samut, Voice of Dissent
1 Scroll Rack
1 Shamanic Revelation
1 Skullclamp
1 Skyshroud Claim
1 Sol Ring
1 Solemn Simulacrum
1 Stalking Vengeance
1 Stomping Ground
1 Summoner's Egg
1 Sungrass Prairie
1 Sylvan Library
1 Taurean Mauler
1 Temple Garden
1 Terramorphic Expanse
1 Thermopod
1 Thragtusk
1 Universal Automaton
1 Wayfarer's Bauble
1 Woodfall Primus
1 Worldly Tutor
1 Worldspine Wurm
1 Zetalpa, Primal Dawn

Link for Tappedout.net deck list