We didn't have an ADR for breakfast, but while we were there last month (early June), one of our MK days had a surprise 8am opening. We already planned to meet my sister and family at the AoA shuttle stop at 8am, thinking MK would open at 9am. However, I was watching TV in our room while getting breakfast ready for my family, and saw the hours for MK changed! YIKES! When did that happen? I had to call my sister's room and let them know to meet ASAP, so that we could try to make rope drop.
I totally understand your concerns. While you can make your reservations and hope for the best, it's disappointing if you are looking forward to that particular restaurant and not be able to go on another day. Personally, I wouldn't do it, but that's just me. I would get REALLY worked up, if something like that happens. You should have seen how panicked I got when I found out about the unexpected 8am MK opening, and we didn't have an ADR ~ LOL!