These need to be requested in advance:
https://www.aa.com/i18n/aadvantage-p...vel.jsp#making
Contact Reservations or visit an American Airlines Travel Center to request a mileage upgrade award.
You may request an upgrade when you book your trip or at any time before check-in.
Side question: Do "American Airlines Travel Centers" exist any more? This makes me think of the off airport ticket offices in cities. I guess it could be a service desk in the airport too?
I've upgraded from J to F on a 321T using miles by asking an agent in the AC before. It might have even been within 1hr of departure as it was an impulse buy as I was walking out of the FL to head to the gate.