I am not asking this to be transphobic or anything but I had this debate with myself at 2 o’clock in the morning and every time I remember it I can’t focus.
On one hand, it is what they want. Let’s assume it causes no harm to them or any unforeseen circumstances.
On another hand, it would erase their identity as trans people. At the extreme you could consider it a genocide, since turning them into what they want would mean there is no more trans people and their unique identity is erased.
I mean, unless you also retconned their past it wouldn’t really erase their identity as trans people. The bigger problem would be that in the process you’d out a bunch of people. Also, what would constitute “turning into a gender”? Gender means different things to different people