For all pre-ops: do you use change rooms, or put off working out/swimming etc until after the surgery? This is an issue I imagine would be difficult for even carry letters to cover.

Yup, sure do.
Went swimming this weekend; paid our $10 entry fee, boyfriend strolls into the men's changing rooms, I wander into the female changing rooms. Turned my back on the other women in there, got changed into my swim gear, walked out to the pools.
No drama.

I usually just jump into one of the bathroom stalls (in the girls room of course) and change quickly.  Seems to work fine, and I think alot of people do that actually.

I use mens bathrooms and mens changing rooms for clothes, but I won't swim or work out in gyms until after top surgery

When there are no stalls available, changing rooms = hell.

One of the major reasons I don't go swimming.

Though my preferred excuse is a vague mention of a (non-existant) aversion to chlorine, and changing the subject.

I was a regular at the local Navy base gym prior to my transition. After my transition, I switched to the women's locker rooms/showers and no one even noticed.

Karen

When I was pre op I use to swim at a place that had covered changing rooms. They looked like large voting booths. A lot of woman used them. It wasn't because they didn't have the right equipment but some were a little shy about being naked in front of others.

I consult to various mining compies. One place it is mandatory to have a shower as it is a Uranium mine. Great. Not a typical shower. About 5 poles with about 4 shower heads on each pole in an open plan layout. So here I am a 58kg, hairless, belly pierced, budding individual surrounded by hairy beer gutted miners. Neless to say I just try an get the hell out of there as quickly as possible

My local swimming pool is unisex.  Everyone has access to their own mini changing room.  You can go in as a boy, and go swimming as a girl and no one will notice.