There has to more to this than simply editing the regions in the province_manager array. If that were the case simple region switches should work out, where you change all managed regions of a province from one faction to another:
I tried allocating Cosentia and Brundisium to Epirus (for a more entertaining Rome start), which should work out reasonably right? Changing region flags, culture, building type and so on as well as switching the faction management of Magna Graecia from Rome to Epirus. I also moved the Roman general from Cosentia to Rome.
I did not touch the garrisons as they appear to be recalculated/reset each turn, after which field battles against Epirus/Cosentia works fine. Garrisons seem ok in the savegame.
- Testing as Epirus: full ownership as expected, I could raise armies, recruit mercenaries and so on in Brundisium. However the AI *still* ninja-builds stuff in MG. A Roman general also keeps trying to leisurely enter Brundisium - but I get not trespass notice, hmm...
- Testing as Rome: Epirus looses its greek provinces in a couple of turns. However Epirus attempts no construction or action in MG. When Sparta attempts a move in on Brundisium their fleet gets stuck in a loop nearby, and keeps trying to attack over and over. After 10 turns of this I declare war on Sparta, and the next turn they conquer Brundisium and Cosentia without issue. Double hmm...
I also tried switching Ibossim and Quart Hadasht between Carthage and Carthago nova. Same issue (AI will not build in its new region, while spending resources on its old region). Changing the management settings will however prevent the AI from raising armies in its old region seemingly.
Anyway to cut a long story short: how do we find which AI settings to change - assuming we can?
Is it even worth trying? Right now using console commands [assuming the exist] seems the simplest way to change the campaign without messing everything up utterly.