@greatninja3
Well, from biological point of view mammals (so humans included) can't change gender- there are few animals (some gastropods, some fishes and few other species) that can do that, but this is different thing. Whoever, there few rare genetic conditions (like AIS or PAIS) when after birth body have problems with handling hormones (mostly androgen) which may end up with taking characteristics of the opposing gender even in later years. It also haves different impacts on the body and depending on the case it may require different medical support.
But there that and chaining gender for mammals after birth would pretty much require some kind of full "rebirth" process as this is how complicated differences between genders are.
Well, there are so jelly fish(like Turritopsis dohrnii) that have ability to "reborn", but it outside of humans/ mammals reach (unless one day some out of the mind genetic manipulation happens- if it's even possible). So you need pretty much goes with flow in those kind of stories and just accept those plot setting things.