From his heavenly perch, God distributes girl and boy hearts to babies about to be born. But a prank by the mischievous angel Tink leaves Princess Sapphire of Silverland with both a male and female heart! Due to an old-fashioned law preventing a woman from ruling, Sapphire must disguise herself as a prince to prevent the scheming Duke Duralumin from coming to power. The plan goes fairly well until a fencing tournament goes awry and the girl is stripped of her royal title. Now, in order to get her old life back, Sapphire must escape from jail, battle dragons, and even get transformed into a swan! Can the girl brawl and charm her way to true happiness?
Another shoujo manga by Tezuka that plays with dualities. Princess Knight's Sapphire possesses both a male and female heart (which are occasionally stolen, landing her squarely on one side of the divide), while Melmo is given magical candies that can transform her from a child to an adult at will.
Neither has aged particularly gracefully, but if you're a fan of classic manga, these might be up your alley.
Princess Knight was one of the earliest shoujo manga, and had a huge influence on subsequent shoujo stories. This influence is especially apparent in works like Utena, that also uses crossdressing and princely female characters as major plot points.
If you liked one of these stories, and you're interested in the history of gender bending manga, or just shoujo in general, check out the other.