Lol i feel so dumb now. That solution is soo simple and i didn't even consider it.
Thank you so much! Im sure i can get that to work.
But im not sure if my units have any free build options left inside their Engineer_ext. i think i hit the limit sadly so im still open to other options.
I was trying to make requirement action inside the units Action.ext, that needed the upgrade, then applied population and cost modifiers. but i cant figure-out how to set it up properly so it actually makes changes ingame.
In that case you can use cost_<resource>_modifier and entity_popcap_modifier to do that. I tested these and they work just fine.