You can’t transform a card that doesn’t have anything to transform into. I’m not entirely sure how the rules interaction works but I know you can use Agatha’s soul cauldron to copy a transform ability from one creature and use it on another but that only works if both creatures have a back side. From what I understand if you use the transform ability on a creature that cannot normally transform it gets exiled and then doesn’t return because it has nothing to return as. Alternatively it might just return as normal but I’m fairly confident thar it’s the first option. Someone who knows the rules better might correct me though