Hang up and call again (and again, and again, it could take many times) until an agent contacts the right desk to force open space on AA metal so you can make the retimed QR flight.

Ask the agent for the specific AA flight you need now to make the connection.
It helps a ton if that connection is less than 4 hours (assuming your prior itinerary was), because the additional fare collect for TSA taxes can add a whole additional complication and HUCAs to get it successfully ticketed, even after space is force opened.

If they give you hassle just HUCA, IMO it’s not worth trying to push the point. The agent will either know to fix it correctly or not.