Metaphor said:
Changing sex--actually changing sex--might be possible in the far future, though I confess I don't know exactly what it would entail. Almost every cell in my body (except my sperm and blood) is XY. How could that become XX? Where would the second lot of information come from?
That is not how it would work. There are fish, for example, that change from female to male or from male to female, without any chromosome changes. In humans, chromosomes determine sex only in the sense that they, under normal circumstances, cause the development of female or male embryos. But it does not determine it in a conceptual sense, or as a matter of metaphysical necessity. Further, in reality, that might malfunction, and if so, at least as far as I know embryonic development might default to female, so there might be a female that is XY. And I mean a female, with a vagina, ovaries, a uterus, breasts, all real, though I do not know whether she would be fertile. But she would be classified properly as a female in the usual sense of the words, and of course a woman, even if XY. She would have normal female levels of hormones, etc., look female even when examined (except the chromosomes), have a female mind, and so on.
See, for example:
https://www.newscientist.com/article/dn16934-girl-with-y-chromosome-sheds-light-on-maleness/
https://www.sciencedirect.com/science/article/pii/S000292970900113X
https://www.cell.com/ajhg/pdf/S0002-9297(09)00113-X.pdf
That was a mutation in the Y chromosome, but the idea would be to deactivate it. Now how do you do that to an adult? I have no idea, and I do not know whether it's even nomologically possible. But
if it is, I suspect this would be kind of like the fish: organs change more or less gradually, including the brain (so, behavior too), until you get an XY female, with the Y inactive.
Whether that would make you a woman is harder to say, given that part of your mind resulted from decades of experiencing the world having a penis, not a vagina, and this goes to truth conditions for the word "woman". But maybe if your memory is erased and you experience decades with a vagina too (suppose people live for centuries or more), that probably would do it.
I'm pretty skeptical that this is doable, but it is definitely not beyond conceptual possibility (i.e., there is no contradiction in saying eventually some men will become women, even counting the meaning of the words).
Metaphor said:
And how on earth would XX become XY?
Not the way to do it, but still, turning a woman into a man seems to present different challenges. No idea how that one would go, without the Y chromosome.