At CYOC:
http://www.cyoc.net/stories/chapter_...lay_previous=1
16-yearold girl step into transformation booth and becomes 20-yearold knockout, but her friend becomes jealous and reverses most of it.
Manga:
http://www.onemanga.com/Rosario-Vampire_II/7/00/
Girl takes pill to become grown up and incredibly strong.
(The good stuff begins around page 23.)