Well, depending on how you want to RP the switch psychologically... You could just RP a legitimately transgender character who goes through great lengths to get their body to match their mind. It would be a simple matter of not feeling comfortable talking about the issue to explain away why it hadn't come up earlier.
"Song dogs barking at the break of dawn, lightning pushes the edges of a thunderstorm; and these streets, quiet as a sleeping army, send their battered dreams to heaven."
Hipparion Tribe (Sagolii)Â - Â Antimony Jhanhi's Wiki