This same thing happened to me and I freaked out and called a friend who already had a Nova interview to find out how they contact you. No, they wont change their minds (hahaha). I got a letter via snail mail a few days later. It gives you a date and a number to call within 2 weeks to confirm (or change) your appointment. Aug. 25th should be fine, it probably won't be past your 2 week deadline, and if you really are worried I'd say give them a call and explain the situation, they're very nice there and I'm sure they'd be willing to help you out.