Since there is no waitlist for award travel inventory on AA, I second the suggestion to use ExpertFlyer to monitor for "Z" inventory availability. Or you could frequently manually check AA.com for first class sAAver award seats on this particular flight, and then call and ask for your ticket to be re-issued in first class for the domestic segment.

Unless AA opens Z inventory, you'll have to stand by for first class at the gate, per hillrider's advice. You should have relatively high priority on the list, effectively as a displaced first class passenger, IIRC. But I would also reiterate the advice NOT to use the word "upgrade" with any AA personnel.