OK here i go into the mix...lol
(I love what I'm seeing for SF)
questions
1) upgrades- i agree with your method most units had to be removed from the front line to be upgraded or overhauled.
-will I as a modder be able to mod this, what units can be upgraded to what units.
for eq. i want to create a string of upgrades for cavalry
"horse cavalry to motorized cavalry to armoured cavalry or air cavalry"
will we be able to "turn off" so when cavalry tech get high enough you are no longer able to build horse cavalry.(this will help keep the clutter down in the build screens)
will i be able to say upgrade to a unbuildable unit, i want to put elites in my mod but i don't want the player to be able to build them (they show up as events), but i want them able to upgrade them, so "Fallschirm Div. 'Hermann Goering'" would show up as a elite paratrooper but then it can be upgraded to a elite mechanized or elite armour unit if the player wants to, but these elites will not be normally in the build screen (keep players from exploiting the elites).
2) will this upgrade be available for naval or air units
for eq. i want to keep a separation between prop aircraft to jet aircraft(plus using a different counter would be nice), so a player can decide to either build a new jet-fighter unit or upgrade his experienced ones into jets.(not just the tech upgrade)
for naval upgrades most ships were not upgradeable per-say but ships did get upgrades in different ways which i would like to be able to do.
eq. most first carriers were converted battleships or battlecruisers
in my mod i have converted carriers as a unit or hybrid carriers.
it would be nice to be able to have an old battleship "upgrade" into a converted carrier or hybrid carrier (but not have these as buildable units).
or downgrade a battleship into a battlecruiser or a cruiser into a AA cruiser.
also HQs i want to create different HQs, with upgrading will HQs be able to be effected, so i can have a infantry hq, mobile hq, armygroup hq. etc.
i hope this makes some sense
so ya as a modder how much modding of the upgrade system will we have ability over or will it be the dreaded hard-coded.