I just rescheduled and I had the option to switch to a date for which registration has started and there is seat available.
So I guess if you want to push it back to next year, which registration hasn't open for them yet, you need to cancel your date and sign up again, when registration open. I guess that means you only get a portion get refunded to you and have to pay again next time you register.
You might wanna email aamc just to make sure.