In general being accepted for being trans is normal in the Star Wars universe, and transitioning is easy as well. Space medicine lets you produce your own hormone treatment and if you want them the surgeries are flawless.

But the Order really isn't keen on their stormtroopers transitioning. It causes paperwork problems, units are all unisex so there's issues there, it's medical care that isn't life or death and therfore "unnecessary". Outside of barrack gossip Stormtroopers aren't told trans people exist, and if they turn out to be trans they are told to shut up and deal with it.

Cue Finn, who after being in the resistance for a little while hears that someone in the resistance has transitioned. There's nothing more to the announcement other than people telling others the new name and pronouns.

Finn is completely blown by it. The only time Finn ever heard anything even remotely about transitioning is when a fellow stormtrooper got offed for attempting it. Finn is torn between being shocked that it's allowed and being jealous. Finn keeps asking more and more questions about it, and then questions about how Finn could do it "if" they wanted to.

Poe and Rey start to catch on to it and ask Finn if they think they may be trans. Finn is initially dismissive, but after being reassured that it's alright Finn says that they might be. Rey and Poe are almost suffocatingly supportive and really excited to help Finn with everything.

Up to you how it progresses from there, but I want supportive fluff.

+Finn and Poe are together
++Finn gets a makeover and cries over how right she look now
++Finn grows her hair out and Poe thinks her poofy hair is so cute and her hair in long braids even cuter.
+++She doesn't want bottom surgery and that's okay.
+++The first time Finn and Poe have sex after she comes out Poe is very very sweet and calls her pretty and his girl and all that teeth rotting stuff.
+++++++++++Poe assumes that Finn wants to change her name but Finn loves it and wouldn't change it for the world.

Thank you!