Can I just change this in the unit infos XML?

And then adjust the promotioninfos according to match?

What happens if the unitcombattype that the promotion infos xml has in it no longer exists?

The most safe way to do so is to make the "dummy" unit combat type and redirect all the links to your current combattype to that dummy. Or it may be unneeded if the game doesn't check this link, then you will just remove the promotion from the game. The easiest way is to try it by yourself.