Clown fish start life as males, and become female in adulthood. Gobies can switch back and forth between male and female. So far, we know of maybe 500 species of fish that can change sex.
I understand people are not fish, but I'm not sure we should be so quick to declare something about people "can't be changed" with enough time, knowledge and science. Sex and gender are both complicated systems with lots of opportunity for unexpected variations affecting seemingly unrelated parts of a person.
It's even possible for your body to have more than one set of chromosomes, it's called Chromosomal Mosaicism and is detected in around 1-2% of pregnancies. Not all of those pregnancies make it full term, and not all mosaics are retained by the foetus, but in a world of billions of people it still ends up being a lot of people who are sexually diverse.
Biology is not simple. Do not underestimate the weird things your body can randomly surprise you with.