If, for example, you build a type 4 government and build up to level 5 regional barracks and then you destroy the type 4 government to replace it with a type 2. You can still recruit those units.

For example, you would be able to build Mercenary Greek Generals in Taras if you first put a type 4 government with a level 5 regional barracks. If you destroy the type 4 and make a type 1 with level 5 native barracks, you'll potentially be able to build both ballistae and those Generals.

It's considered an exploit though as the mod really expect you not to do such things, but you can do whatever you want.