Or, that the duration of transformation is what takes up the energy. Changing a tortoise into a teapot for the duration of the class is relatively low energy consumption; changing it permanently would be very taxing.

Oh, I like this. It fits in quite well with what we've got so far.

I wonder how that works with Animagus transformations, though. Is that a branch of transfiguration? Because we've seen that someone can live in that changed for for years.