And I feel than thats kinda BS since the only thing than should happen its than the units lose some XP due having to get used to the new equipment or models (like switching from Light tanks I to Light tanks II) but I dont find any reason of why I cant do it.
Some people are going to point out than you can still train a new OHQ and let them train on your capital (or behind frontlines), but they take a looong time to catch up on xp with the frontlines units, and I really want to have an option to change the equipment of my units without having to wait for them to get killed and then replace them.
