When people change change an X chromosome to a Y, or vice versa, then they should be able to change their birth certificate.
Until then, your sex is set by your DNA and superficial changes are just that... superficial.
"Look! I had surgery! Now I'm a woman!"
"Dude -- you got your ears pierced. You're still a guy."
More extensive surgery doesn't really get anyone further down the road. You're born a man, or you're born a woman. You are what you are.