I was thinking like if Captain America changed from human to non-human then back, if he would lose his serum powers and become a normal human? since it wouldn't affect his non-human form and the serum is not something that he naturally started out with, but was injected into him, making it possible for the serum to be ejected during or after the shift.

There's also the question if say someone shifted into doomsday to adapt, shifted back to their base, then shifted back to doomsday, would they have lost the adaptations they developed before and ended up just shifting to base doomsday? with the being they shifted to not having anything the user developed themselves.

Prob ig

In the context of Captain America losing his serum powers when returning to a human state, no. He'd likely keep it unless the writers wanted otherwise.

Otherwise, yes their changes would be reset when species-shifting. Beast Boy changes back to the same human regardless of what animal he was or what happened to him. Only exception would be something like missing limbs or something ig

Ben 10, for example, broke his arm in one episode. The alien he shifted into, being Swampfire, despite that alien's regenerative abilities, still had to wear a sling. Once Ben changed back, his arm didn't get any better.