Small update- for anyone who cares- went to the BA Transfer desk when I arrived into LHR from Lisbon who then managed to open a seat up on an AA flight (125 ticket number though) under a different PRN, so I managed to arrive 6 hours late but into LA tonight.

Funny thing is I still now have the flight tomorrow on my other PRN, called AA who said not to worry and it wouldn't cancel my remaining/return flights when I miss it. Still a little concerned but time will tell..!