I personally would really like to know why they need a bathroom stall to change in at all.
Not choir related, but when I was in middle school gym, I changed in the bathroom stall. I was horrendously bullied for being an early bloomer, and I was not going to take my clothes off in front of girls who bullied me about my body - I didn't care if people yelled at me, but they were not going to get a chance to steal my day clothes or pull down my underwear in front of the entire gym class. I usually used the handicapped stall, because we didn't have any handicapped students in my class, and I could move the most quickly in there. In high school, there were unused, but curtained shower stalls for me to use.
I think the generally polite thing for handicapped stalls is to take the unoccupied regular stalls first (unless circumstances call for the larger stall). If you know you're going to need awhile and it's a public building where a handicapped person could easily need the space, you should try for a regular stall, even though the handicapped one tends to be more comfortable. But if it's the only one left, it's the only one left.
As for access... it can get annoying. I have atypical migraines that leave me with temporary paralysis. I've had to go through stores in wheelchairs (walking through the store just fine, exposed to a migraine trigger, and end up unable to move my legs. If I'm lucky enough to have a companion, they'll get the store wheelchair for me. If not, I have to stand there until it passes), and it is often incredibly inconvenient. I'm lucky that I only have a transitory disability, and I've never been paralyzed for more than a few hours, but one of the worst instances, when the migraines first started, ended up with me having to be carried out of a building because they only had stairs to the floor I was on. It was in college, so I assume they'd re-arrange classes for disabled students, but a student in a wheelchair couldn't have attended classes at my school. It was a beautiful Victorian vintage campus built on a steep hill - there were stairs everywhere. Even with the ramps they added in the 70's, a student in a wheelchair would have needed a companion or amazing arms and fantastic brakes, otherwise they would have ended up somewhere around the train station everytime they tried to get to upper campus.