If it makes you feel better I had to reschedule mine twice. First time was because the AF couldn't schedule my enlistment in time to get me to the September class, and then I got bumped to January. Then I had to personally bump that back due to a medical thing that came up and I'm working through at the moment. Not that I am happy about it but that was the situation I found myself in. So I am waiting on my new date. In both instances they didn't give me a hard time about it, although they might try to push you a bit since they do want to fill classes up.
bb17's post in Flexibility of OTS classes was marked as the answer