This one has a lot of charm! I like the concept and would like to see a follow up made.
I've never used GB Studio before, so I don't know what your limitations are (or how much time you were able to spend on the game.) But it would be fun to make things a little less linear.
Right now the puzzles are very straight forward; "Only A can go here!" --> "Oh, you're A? Okay, you can go here then.. :D"
Giving the transformations different properties would be great. Like some can shoot or push boxes or jump over low fences. Things like that would make the puzzles feel more organic versus just grabbing the right 'key' for a 'lock'.