From the Winter 2018 schedules BA and AA are swapping frequencies with AA taking all the LHR-DFW (and adding a 5th frequency on some days), and BA moving to take over one of AA's LHR-MIA frequencies. Might be worth checking if you've been rebooked onto an AA jet from T3 anyway.
https://www.routesonline.com/news/38...as-of-09may18/
https://www.routesonline.com/news/38...ervice-in-w18/