First and last it should be said that changing genders is a radical decision and it is not likely that anyone would undertake such a process lightly or frivolously. But to be honest, I'm not sure how to properly understand or appreciate such a choice. (That's intended as an admission of ignorance, not a judgement.)
On the one hand, I can't help but have compassion for someone who feels so fundamentally out of place that they want to change bodies in order to be themselves.
On the other hand, in Islam there are various restrictions on certain kinds of body modifications and cosmetic procedures (e.g. tattoos, wigs, hairpieces, dying hair to look young, filing teeth etc.) to avoid vanity. And there is a general principle prohibiting "changing the creation of Allah" (taghyir khalq Allah). This restriction is certainly not absolute and I've never seen its limits precisely defined, but the principle suggests to me that we should be happy with our bodies as they are naturally and not mess with them too much without good reason.
What is "good reason"? That's not quite so clear.
In any case, there are both Sunni and Shia scholars who have given fatwas permitting sex-change operations if necessary.
the men will look like the women...
the grass is always greener...