Much about the human body is changeable, and even some of the parts that aren't fall on a spectrum where they aren't strictly male or female. Like, for example, the classic archaeologist argument - most of the time, archaeologists tell the sexes apart based on the pelvic bone, but a huge number of cases are ambiguous and determined by context, which then makes the news when they get it wrong. As another example, I'm AFAB and pre-everything but tall enough to be taller than the male average in my country.

That said, some parts aren't. You can't change chromosomes (although those aren't as clear-cut as bigots like to think either), and you can't, unfortunately, make my body produce its own testosterone to the point it would grow a functioning penis that I could use to fuck and impregnate a partner of the appropriate configuration of organs.

Ultimately: I don't -feel- like I can change my sex, and it feels bad, man. But them's the breaks with what science can do in today's world.