Originally Posted by
anabolism
The agent was wrong, you are correct. "Marketed" means the airline who's code is on the flight, and has nothing to do with which airline issued the ticket. As long as the flight is both operated and marketed by AA (also called an AA prime flight), you can use an SWU to upgrade it.
Thank you for confirming my understanding. I’m actually flying LAX-JFK-LHR and did search each individual segment and whilst LAX-JFK was A0, JFK-LHR was indeed A1, and I could also see the SWU link on AA.COM when searching on that flight, so availability is definitely there for that leg.