Kim,
I know a little about the education system (at least here in Houston) so I can empathize with your frustration about having a plan and keeping to it. Unfortunately, most AN treatments are not life threatening and, as you said, not urgent so changing the date isn't uncommon. I wouldn't go so far as to say it's typical, but it's not unheard of for it to be rescheduled because something else came up.
It sounds strange that you don't want to postpone
BRAIN SURGERY any longer becuse it might interfere with something else, but I understand where you're coming from..
It very likely won't be changed again, but you should have at the back of your mind about what you're going to do if it does change again... After you plan a surgery date, very much is out of our control about timing, recovery, etc.. If you can, force yourself into the mode of adapting with the changes because the reality of it is we don't have a lot of control over that. Being in education, you also understand that sometimes things just change and the best you can do is roll with it and adapt the best you can.
FYI, come here to vent anytime you want.. Lord knows I've vented on here enough.
Regards,
Brian