Honestly the abilty to change the transformation of you or one of your companions from Diosisia in the Lunar Forest, is completely pointless, with it completely resetting a characters height to 1.8m. Realistically that means, once you picked the transformation, it is essentially as locked in as the permanent stats of the characters.
While I get that it probably shouldn't be entirely free, maybe being able to return to a transformation type you used before at maybe half the height(but min.1.8m), or maybe down one stage or something like that, whatever you feel makes sense. That way there would still be a cost to it, but it would not be completely pointless either by resetting all your progress entirely