Correct me if i'm wrong, but Sentai & Sensibility doesn't actually have advancement rules right, 9th Level Games team?
Correct, the polymorph system isn't design for "leveling up." However, you are encouraged to change out team attacks, machina combinations, or really anything else about your character you want for thematic or story purposes.
Thanks a lot for the tips, 9th Level team! I mean changing colors happens too every so often in Power Rangers comics.