2 years ago I missed a flight, OSL-LHR-LAX-SFO in J, with the LHR-LAX sector upgraded to F on AA using a SWU (AA ticket) , it was an error fare at 600€,
When I arrived at the airport the plane was about to take, no BA staff as they use contracted staff in OSL,
I called BA to explain the situation, and they quoted me 7000€ to change my flight, I hanged up and called the EXP line, they reroute me via JFK and then the transcend to SFO, all in F, for 0€,
Even if I screwed up, on a very cheap J ticket, AA didn't let me down, not sure they would still do it, best part of being an Emerald with AA is the EXP line