At least, you could allow to reassign them to a more prestigious position. The prestige here would be determined by number of battalions, with some bonus for home countries. So you could reassign a good general from colonies to command a main front at home, and they would happily accept it. And it would be either forbidden in the other way (easy to do), or, preferably, it would be possible but with some sort of penalty (malus to opinion of general's interest group, or if the said group is already unhappy and powerful enough, chance of uprising).