@pop91
Hypothetical:
Say there was a perfect sex change in the future where every single cell could be recoded as male for a trans man, would you then accept transmen as men? or would they need to be born male even if their entire physicality and biology were changed?
(To be clear this sex change would hypothetically naturally dissolve breasts and naturally create a penis as well as ALL other physical changes)
Yes it would be a man. But it would not be the same person as before the change.
This is like the question people sometimes ask, "What if you woke up tomorrow in a man's body? How would you feel?"
Such questions and yours, OP presuppose that there is something that makes you who you are independently of all changes to your body. Usually this is thought of as a soul of some sort.
We owe this idea of an independently existing soul, more or less, to Descartes. It is intellectually enticing, but quite wrong.
Of course some bodily changes ageing in certain ways, even amputations, etc. will still leave me the same person. But some aspects of my physical body are necessary for me to be me, amongst which, my sex.
Another way of saying this, perhaps, would be to point out that changing sex is not just a scientific impossibility: it is a conceptual , or, if you like, metaphysical impossibility that a human being changes sex.
So, whoever wakes up tomorrow with a penis and all the trimmings, it will not be me. Why? --Because I am a woman.
Likewise, OP, with your thought experiment. The person after the change would not be the same person as the one before the change. Why? --Because they have different sexes, and being a certain sex is an essential component of who one is.