I am playing GBA version of Super Mario Advance 2: Super Mario World. How to get normal koopas back?
Solution 1:
According to the Super Mario Wiki,
After the player beats every special level in the Special Zone, a game aesthetic change called Fall occurs, where the world map obtains a different palette and some enemies get their sprites changed. Once Fall is activated, these changes cannot be reversed unless the save file is deleted and a new game is started.
So, it sounds like you are stuck with the new sprites, unless you start over.
https://www.mariowiki.com/Fall