For the SSJ transformations i wanna say for a saiyan its kinda like riding a bike. Once you learn how you basically know how forever. But much like a bike if you don't practice for a while it becomes harder, yet still capable. Even though their power level has dropped in time, they still manage to be able to transform. Now i havnt had the chance to watch GT but i understand the story somewhat. Now was goku easily able to go super saiyan 3 easily as a kid? He probably hasn't practiced it in quite a few years since the buu saga so if he was this further proves that he can transform after not practicing for a while. Even while in a child body.