Wouldn't people who had the whole "reassignment" thing done require a level of ongoing medical support that would make military service impractical?
That's my understanding, though I have to admit that it's well outside of any of my areas of expertise. Without regular hormone treatments, my understanding is that the body tries to heal back as much as possible toward its original unmutilated condition. I've heard/read, I think, though I don't know how true it is, that regular surgeries have to be done, to reverse some of that healing that occurs even with hormones.
In short, my understanding is that a post-postoperative transgender body is unstable in that form, and requires ongoing hormone and surgical treatment to keep it from reverting back toward the morphology of its true sex.