Hime-chan no Ribbon(1990)

Hime-chan no Ribbon

Princess Erika of the Magic Kingdom must test out a magic item that she created for a year on her human world doppelganger before she can qualify for the throne. Her look-a-like happens to be Himeko, a tomboy who worries that her boyish looks and attitude are ruining her chances with the boy she is in love with. Erika gives Himeko a ribbon that allows her to transform for an hour into anyone she wants, but if she exceeds the time limit, or if anyone finds out, she will be stuck in that form for the rest of her life.