Comment 8 for bug 895223

i think a better way to upgrade units would be

UnitToBeUpgraded=GTNK ; says which unit is going to be upgraded
UnitToBeUpgradedInto=GTNK1 ; what the upgraded units turn into, all of this team's GTNK are going to be turned into GTNK1 when the upgrade is used
RemoveOldUnit=yes/no ;which says once the unit has been upgraded, only the upgraded version can be bought, not the un-upgraded version

then if you want to make a minor upgrade, like making all of your units immune to radiation or changing a primary weapon, you just copy the old unit, change it's name and adjust whichever tag(s) needs adjusting. this would also reduce the amount of new tags required, and make it easier to make more radically different upgrades. Of course this does prevent you from making multiple upgrades for the same unit.

you could also have

IsUpgradeUnit= ; goes on the unit itself, means the unit cannot be bought until it is unlocked by the upgrade

the original two tags could also work as corresponding lists so for example
UnitToBeUpgraded=GTNK,MTNK,FV
UnitToBeUpgradedInto=GTNK1,MTNK1,FV1

the GTNKs become GTNK1, MTNK becomes MTNK1, FV becomes FV1 etc.

and maybe for the sake of ease have
UpgradedUnitCameo= ; which replaces the upgraded unit's cameo, since a lot of modders would probably use the same image for both the upgraded and non-upgraded units, this tag would save them from having to clone the voxel, and add new art entries.

perhaps the upgrades themselves should be superweapons, which have cost= (so you pay for the upgrade superweapon, if people want the upgrade for free, they can set the cost to 0, or have cost=0 as the default), it would use Type=Upgrade, which doesn't have any effect as a superweapon and the UnitToBeUpgraded=, UnitToBeUpgradedInto=, UpgradedUnitCameo= and RemoveOldUnit= tags go onto this superweapon. perhaps it would be a good idea if those tags could also go onto buildings and building upgrades as well, so your could have a research center which gives you those upgrades instantly instead of waiting for the superweapon, or maybe even put the tags onto units and infantry.

EDIT: perhaps if the upgrades were similar to elite weapons and abilities, so
UpgradesUnit= ;this is the unit that will be upgraded
UpgradePrimary= ;upgraded unit's new primary weapon
UpgradeSecondary= ; secondary weapon
UpgradeAbilities= ; same as EliteAbilities=, but goes on the upgrade and effects the unit. unit receives these abilities when upgraded

and then some tags like the 'veteran rules' in the [general] section, but these also go on the upgrade itself. if these aren't used, it defaults to using the veteran rules in the general seciton (or if they're not listed in UpgradeAbilities=, they have no effect)

UpgradeCombat=1.1 ; multiplier to damage
UpgradeSpeed=1.2 ; multiplier to max speed
UpgradeSight=0.0 ; multiplier to sight
UpgradeArmor=1.5 ; multiplier to Strength
UpgradeROF=0.6 ; ROF delay multiplier
UpgradeCap=2 ; maximum Upgrade level that can be obtained. unit can only recieve his upgrade this many times.