So, this is a very old one, but the anime series "
Kamikaze Kaitou
Jeanne" had somewhat of an AP TF for a loli demon character named "
Myst". She had these demonic candies that she would use to possess
people or eat them herself to increase her power. In the
40th
episode, she becomes really desperate to beat
Jeanne, the
protagonist, that she overeats the candies and grows into a monstrous, yet
mature-looking demon form. The process is slow throughout the middle of
the episode and isn't entirely shown, but it can be seen briefly in a couple of
different scenes.
(8:25, 9:05-9:22, 10:50-10:59, 13:51-14:03)
https://www.youtube.com/watch?v=lbmruO_Gdo0