You rarely see planned switches from one tech tree into the other, because the lack of upgrades is so much worse than the gains from another unit composition.
So if I had the magic wand of balancing, this is how it would look:
New Mechanic: Threshold - As soon as Upgrade X is researched, the unit gains a bonus.
Introduced to be able to modulate the strength of a unit depending on the phase of the game. Also modification of the upgrade-effects are way underutilized.
TERRAN:
Barracks:
Marine
Damage from 6 to 5 (+1 to light)
Threshold: +1 Bio Weapon - Damage changes to 6
Reasoning: The marine is essentially fine. It's powerful in numbers, but very vulnerable. You clump, AOE kills you, you spread, melee does quite well. At the same moment, the large numbers, high dmg and range would make a hard to give it a good threshold ability without it being utterly broken.
So I nerfed the marine slightly in the early game. This is also to reduce the potency of bunker rushes vs Zerg (lower dmg vs spines and queens) and makes the marine less of a catch-all defense early. The threshold brings it just right back.
Marauder:
Damage from 10(+1)+10(+1) vs armored to 10(+0)+10(+2) vs armored
Threshold: +1 vehicle armor - gain +1 base armor
Reasoning: The marauder in the bio composition should be a tanking unit. Also the bio-dmg is very high. In the beginning, marauders are seen less and less, so I wouldn't nerf. Later the marauder should have more of a niche. That would be tanking, adding armor as a threshold. Currently, the marauder is weak vs lings and zealots (which are destroyed by marines). Buffing armor would really up the utility vs those units and thus the damage was shifted for the late game. On even upgrades, the damage vs light decreases from 10 to 7 over time.
Reaper:
Cooldown of D8 packs tripled.
Nitropack changed from + speed to an activatable ability:
Increases speed to 10 and negates 50% of incoming damage for 6 seconds. 45 seconds cooldown.
Threshold: Cadeuceus Reactor - gains passive health regeneration of .5hp per second
Reasoning.Well.. nearly everything you do to the reaper is an improvement. You cannot really improve it in the early game, else there will be rushing issues like in the early age. Nitropack just doesn't really cut it in stable situation and therefore I changed it to an active ability which is kind of an 'get out of jail free' card. You can also use it to rush in, therefore the DPS vs buildings had to be lowered. Oh.. and regeneration is really nice on a hit&run unit that you might use at one point, then get out and wait for another opportunity.
Ghost:
Snipe damage changed from 25+25 vs psionic to 20+30 vs biological+40 vs psionic
Snipe energy cost increased to 50 from 25
Snipe can now target all units
Snipe has a 2 second casting time - damage and energy cost occur at the end of the spell. Should the target get out of range or energy decreases below 50, the spell is canceled.
Snipe range increased from 10 to 11.
Mobius reactor replaced by Commando training.
Commando training increases starting energy by 25 and enables the camoflage ability
Camoflage ability:
When not firing or moving, ghosts cloak free of cost with a 5 second fade time. Cloak is broken, if the ghost moves, uses his regular attack or is attacked.
Active cloaking field will not drain energy when camoflage is active.
Threshold: +3 weapon - base damage changed from 10(+1) + 10(+1) vs light to 15(+1) + 5(+1) vs light
Reasoning:
I always thought the ghost cloak to be... clumsy. Also the snipe is a troublesome thing, running around and sniping on the run is the opposite of what I would envision ghosts to be. Combine new cloak with new snipe and detection will be important when facing ghosts. Simply because you can place ghosts in strategical points to spot and - if undetected - remove high priority targets from a passing army. The ghost-HT dance will be a new one. The protoss player has 2 seconds to feedback, before snipe goes off, while the terran tries to distract and hide ghosts.
Factory:
Hellion:
Infernal preigniter adds plus 10 vs structures
Attack cooldown increased from 4 to 4.5
Threshold: +1 bio armor - Damage taken from splash reduced by 30% when not the primary target.
Reasoning: In a mech setup, the hellions have a tanking role due to they cheap cost and low production time. They problem is that they deal no sustained damage and are wrecked by AOE damage (colossus especially). The threshold ability gives them better tanking ability without an HP increase that would help them in their current role.
Siege tank:
Siegemode damage increased from 35(+3)+15(+2) vs armored to 70(+7)
Siegemode attack cooldown increased from 3 to 4
Tankmode armor increased from 1(+1) to 2(+1)
Siegemoder armor decreased from 1(+1) to -1(+0)
Threshold (Tankmode): +2 armor AND Building Armor - Gains +20hp in tank mode only
Threshold (Siegemode): +3 attack AND Concussive Shell - Splash damage slows the movement speed of affected units by 20% for 2 seconds.
Reasoning:
The tank is too weak. For it's mobility, its MUCH too weak. It's the main reason why mech sucks in many ways. Mech should be strongly dependent on positioning. Removing the penalty vs light is just need. The cooldown change means that the new DPS is even slightly lower than the old vs armored, but much more frontloaded. That enables sending in small groups spread first to bait the volley and utilitze the change against the tanks.
Thor:
Strike cannon replaced by weapon refit. Immobilizes Thor for 15 seconds (rebuild animation from the campaign used).
Upon use, changes the air damage from the current to 3x10(+1)+5(+1) vs armored, no splash, 3.5 second cooldown.
Threshold: +3 armor - Gains passive health regeneration of +1hp/sec
Reasoning: Strike cannon plainly sucks. A possible solution would be a change to refit (which could be the threshold ability too if needed), which also allivates the broodlord vs ultra problem abit, as the thor would be really expensive, but decent against both. The refit could also have a resource cost involved (e.g. 50 gas).
Starport:
Viking (assault mode): Attack damage decreased, attack cooldown decreased (better vs 0 base armor, equal vs 1 base armor, worse vs 2 base armor)
Uses the better of vehicle and ship weapons
Gains +1 armor, loses hp (break even should be 7-9 dmg)
Threshold (Assault mode): + 1 vehicle armor - transformation time to air superiority fighter decreased by 50%
Threshold (Air mode): +1 ship plating - transformation time to assault mode decreased by 50%, deceleration rate increased
Medivac:
Air weapons decrease the unit drop cooldown by .15 seconds per level
Threshold: +2 ship plating - Acceleration increased
Banshee:
Threshold: +2 ship attack - Range increased by 1
Battlecruiser:
Immune to neural parasite
Shoots while moving
Ground attack changed from 8(+1) to 7(+2)
Ground range increased by 1
Weapon refit renamed to Yamato Cannon
Threshold: +3 ship attack - Can utilize anti-air and anti-ground batteries simultaneously
Raven:
Seekermissle renamed to Hunter-Seeker-Missile
HSM range increased to 9, HSM speed now depends on the distance to the target (5.5-0.5*distance)
HSM duration increased to 12
Durable Material upgrade removed
Autoturrets timer removed. Autoturrets start with 200 energy and expend 5 energy per attack (energy recharge rate: .25/sec). When energy falls below 5, the turret autodestructs.
Threshold: Behemoth reactor - Energy maximum increased by 30
Bunker:
Buildtime increased by 5 seconds (Just had to be)
Neosteel frames now allows Hellions, Siegetanks and Vikings to be loaded.
SCVs set to autorepair can repair from inside.
ZERG:
Creep recedes now 30% faster if not supported by tumor.
Creep decreases burrow and unburrow time by 50%
Hatchery tech:
Overlord:
Threshold: +1 flyer carapace - Instead of being killed immideately, the overlord gains a life timer of 3 seconds when its HP are reduced to 0
Zergling:
Movement speed pre-ling-speed increased by .15
Threshold: +2 armor - Supply cost for zerglings reduced from .5 to .25
Queen:
Transfuse healing reduced from 125 to 70
Transfuse energy cost reduced from 50 to 25
Inject larva energy cost increased to 30 from 25
Threshold: +2 melee attack - off-creep speed increased to 2
Baneling:
Gains .3 movement speed while burrowed on creep
Threshold: +2 ranged attack - upon death creates a radius 1 pool of acid which deals 4 dmg per second for 5 seconds
Roach:
HP decreases from 135 to 120
Armor increases from 1 to 2
Threshold: Adrenal glands - Gains 25 hitpoints
Lair tech:
Hydralisk:
Rangeupgrade increases range by 2 up from 1
HP increased from 80 to 85
Threshold: Roach Speed - When attacking targets at range 3 or closer, damage is increased by 4
Mutalisk:
Slight increase in acceleration
Threshold: Overlord speed - When out of combat for 15 seconds (15 seconds passes since the last damage taken or the last attack), regenerates 8 hit points
Corruptor:
Threshold: Infestation pit - corruption now reduces target units attack range by 1, can be stacked up to 3 times (dmg increase does not stack)
Infestor:
Fungal growth no longer immobilizes, instead it silences and slows by 90%
Units affected by fungal growth gain a fungal growth marker
Each fungal growth marker reduces damage taken by .5 per tick and slow by 15% when fungal growth is applied first (this happens before the marker is added)
Neural parasite is now an innate ability
Neural parasite research replaced by pathogen mutagenesis:
100/100, 120 seconds - removes all existing fungal growth marker
Threshold: Hive tech - Neural parasite can now be casted while burrowed
Hive tech:
Ultralisk:
Gains attackrange of 1.5
Hitpoints decreased from 500 to 450
Base armor increased from 1 to 2
Threshold: +3 carapace and roach speed- armor increased by +2 against ground attacks from the front (120 degree)
Broodlord
Threshold: Ventral sacs and +2 flyer attack - spawn extra broodling now on a 2 second cooldown even while attacking
PROTOSS:
Warpgate cooldown increased to build time +15 seconds
Gateway tech:
Zealots:
Build time decreased by 4 seconds
Charge autocast removed. When activated during attack move, the zealot charges the next target within range 10 and scores at least one hit.
Charge can now be activated during move and covers distance of 8.
Zealot shield armor increased by 5 during charge.
Threshold: +2 attack - Gains +2 damage vs light
Stalker:
Build time decreased by 2 seconds
Damage increased from 12(+1)+2(+0) vs armored to 12(+1)+3(+1) vs armored
Shields increased from 80 to 90
Threshold: Dark Shrine - Cloaks for 2 seconds after blinking
Sentry:
Damage increased from 6 to 8.
Forcefield is now a channeling spell.
Energy cost changed from 50 to 25 +1.5/second
Hallucinate is an innate ability
Hallucination research changed to Psionic Hallucination - an upgrade to Hallucinate
Psionic Hallucination:
Imaginary units created by this spell are undiscernable by true sight until they fall below 70% of their shields.
Threshold: +1 shields - Sentry gains +20 shields
Dark Templar:
Warp-in time reduced from 5 seconds to 3 seconds
Threshold: Observer speed - Gains .5 movement speed while detected
High Templar:
Feedback no longer deals damage to mechanical units
Storm damage increased from 10 dmg per tick to 15 dmg per tick for the second half of the duration
Threshold - +3 attack - High templar gains range 4 detection when over 100 energy
Archon:
Threshold: +2 shields and Shield battery - Shield regeneration is increased by 200%
Stargate tech:
Phoenix:
Threshold: +2 air Weapon - Graviton beam reduces targets armor by 2
Voidray:
Range after locking on a target increased by 1
No bonus vs massive
Beam ignores armor starting from charge level 2
Threshold: Extended thermal lance AND +1 air weapon - Gains +1 range
Carrier:
Immune to neural parasite
Spawns with 8 interceptors
Acceleration greatly increased
Interceptor launch range set to 8
Interceptor operation range increased to 15
Interceptors will recharge shield and repair after every second attack, while within range 8 of ANY carrier
Threshold: +2 air armor - carrier gains hardened shields with a 10 dmg threshold
Mothership:
Supply cost increases by 2
Immune to neural parasite
Base shield armor increased from 0 to 2
Weapon range increased by 2
Fires while moving
Vortex replaced with Stasis
Threshold: +3 air attack - Gains the ability to shoot at untis in stasis
Robotech:
Observer:
Threshold: +1 shields - no longer discernable while not detected (no more ripple effect)
Immortal:
Base shield armor increased from 0 to 1
Threshold: +3 shields - hardened shields reduce damage to 8 down from 10
Colossus:
Base range decreased from 6 to 5
Extended thermal lance range bonus decreased to 2
Movement speed reduced to 1.875
+5 Shield armor vs Air-to-Air attacks
Threshold: Anion Crystals - Range increased by 1
Research at the cybernetics core:
Shield battery (50/50, 60 seconds)
Enables each nexus to transfer its shields to a friendly unit within range 12. Cost: 2 points of shield for each one point replenished. Transfer at 25 target-shields per second.
Increases the nexus shield regeneration by 100%.
This ability can be cast manually. Enabling Autocast will target the unit with the least shield points.