Day-of-travel issues are on the operating carrier, so AA would be responsible for accommodating you should something go sideways with their flights. That said, as the ticketing carrier AS is capable of helping out, too, if they are so inclined, but generally cannot "force space" on AA equipment. So, to answer your question, were award space available, AS could make the change, but more than likely it will not be, so you will be at the mercy of AA. Pro Tip: if you need to reach AA, use the phone number you received with your 75K upgrade cert--that goes straight to the EXP desk.