My latest find has been the 2011 anime Un-Go. Un-Go is set in Tokyo in the near-future and is about a Detective and his mysterious companion solving crimes and exposing human souls. Or at least that's the very basic plot according to wikipedia, I've never watched the series. The character of Inga has two forms in the series, a Young boy and an Adult woman. Inga switches between these forms quite regularly.
Inga can be seen transforming at about 0:38 in this video:
https://www.youtube.com/watch?v=tpxHwZ0nLZQ