There seem to be several words missing or out of sequence in your description, leaving me unsure who changed, how and why, and whether you're referring to a fanfic or a real Oz story.

But the main character in L Frank Baum's second book, Marvelous Land of Oz, is the boy Tip, who turns out at the end to be Princess Ozma, who had been transformed by the witch Mombi when very young and raised as her adopted ward.

There's also a Ruth Plumly Thompson book, Speedy in Oz, where the boy title character turns out to be a dead ringer for a tomboyish princess nicknamed Gureeda Book, and a plot is hatched to palm him off on a giant who's demanding the princess as a slave. But there's no transformation there, and the only subterfuge is in attaching a false braid to his hair to make it look like hers.

Assuming that's not what you're after, you might want to make your description more clear.

Eric