Thanks for your response, ford! That's great that your workplace already has a plan for you and your coworkers. The place I work at does not have HR directly in the building. It's a chain store. Regional HR is around here somewhere, but I would have to call them. It seemed unnecessary to do that in this particular case (although I would have if I still worked at my last job and had HR in the building); really the only reason I would consider calling regional HR is if I had some kind of harassment that could not be resolved at the store level. Nothing much really needs to be changed at my job to deal with my transition. I am only telling them to keep them from being confused when it looks like I am going through puberty at an abnormally old age.
I guess "supportive" is the wrong word. Really I just want people to remain professional and keep work strictly work. But I have one supervisor who tends to allow her personal non-work-related opinions slip out on occasion about all kinds of subjects. I am just hoping I don't end up having issues with her.
That being said, this is just a night job I have to get myself through grad school, so I shouldn't overthink this too much. I am probably worrying over nothing, anyway.
Also, thanks, Simon. That was very helpful. I will tell my boss ASAP, and hopefully everything should work out ok by the end of my "grace period".