Reading through this thread I'm not sure if the desire is for a character who is transgender, and how to play it, or how to bring the stigma and emotional feel of being transgender to a character.
Given the cost of the potion, or spell* a level 1 character probably won't have enough to have it, so it might be worth some to think about it if you're dedicated to the idea. There is a lot to be said about the personal implications, but 90% of the time Adventurer's wont be anywhere near their home or anyone from home without either GM or Player specifically invoking it. In most of Golaron your sex/gender won't matter because magic is a great equalizer ("I don't care if you're male, female, or sprout 3 of each genetalia once every 3 months, all I want to know is if you can kill the thing eating my sheep!") so unless you work with the GM to make it an issue, it probably won't come up.
*Mechanically Bestow Curse imposes a Permanent change to your character with some rather harsh maximum effects, but it can explicitly do lesser things. With GM approval one 3rd level spell and a willingly failed Will save and you've just changed Gender (sex? not sure which fits).
On the flip side of that, if you want the stigma of a real world transgender character in Pathfinder then perhaps someone who wants to be another RACE will fit the RP better. There's a few ways to change your gender in Pathfinder, but I don't know of any to permanently change your race without being reincarnated into another body. Sure your human could get surgery to have their ears pointed, or cheeks thinned, etc. but there's nothing that will permanently change them to BE an Elf.
So for the example human to elf might LOOK elven, but there are ways to divine if someone is an elf or not, and some powers or items may need you to actually be an elf to function right. (Which fits the feel of a transgender who may want to father/birth a child but lack the parts to do it). This also brings up the question of how said person is viewed by either race's xenophobic populace. Do human elitists view this person as a traitor, abandoning their native race? Do the elves view them as a poser and reject them from their community, or welcome them as a convert?