To make further testing simpler, and so you people can help me better, I switched to Native.
So I created a test unit called "Swadian Paladin"; it is based of the swadian knight, but has higher stats, higher skills, and a level of 50.
I made it upgrade from the swadian knight.
I checked it in-game: I had to press ctrl+x 12 times (so it needed a bit less than 12000exp) to make it upgrade.
I closed the game.
I modified the default Swadian Knight so it now is level 5 (from default level 25)
I checked back in-game, and.... I still need to press ctrl+x exactly 12 times to get a Paladin.
What am I missing here?
EDIT: Oh ****, I know. I need to start a new game.

Ew. So most troop changes are save-compatible but not the experience-to-upgrade thingy? Damn.
Is there any way to make it so?