That would only be realistic with certain factions though... I mean you wouldn't find the Romans recruiting hastati in the time of marius' reform of the army, so it would be ok to remove lower tiered Roman units once they reach the next tier.. (which uh, already happens) But other factions that didn't have full time proffessional armies like barbarians and eastern factions and such would levy every available man they could in times of need, including peasants with no training, arming them with a spear and a shield and throwing them into battle... So for most factions leaving the first and crappest tier of units in after they unlock the next is realistic in my opinion...