I MAYBE found a way to change to owner of a region and I want to implement this feature soon in the SaveParser.
What I managed so far is that the game won't crash if you inspect the changed region by "gather information" and the
garrision is set up with the new factions units types.
But before I release this new version I need some people who helps me to test this feature and report any weird behaviours in the game.
If you wanna join the beta-test, apply here.