In the first game I ran, we had a player whose character was a cursed werewolf. She could voluntarily take on the form of a large wolf. But when the curse forced her to change, she was a humanoid wolf with horns.
.... So.... I think it's up to you how the change happens. I like how someone suggested Modular Abilities. So, taking his advice, I'd just run it as Modular Abilities, Beast Change or True Shapeshifting.