Hime-chan no Ribbon

Princess Erika from the Magic Kingdom must conduct a year-long evaluation of her newly-developed magical artifact on Himeko, her human counterpart, in order to be considered for the throne. Himeko, a tomboy who is anxious that her manly characteristics and appearance are impeding her chances with the boy she fancies, is the test subject. Erika bestows upon Himeko a ribbon that enables her to transform into any desired shape for an hour, but exceeding the time limit or uncovering her actual identity would lead to permanent confinement in that shape.