As the switch back to a 777 (for both AA50 & AA51) only appears to be for 2-17 Dec I'm not sure there's any reason, yet, to believe that there's a permanent change on the way. DFW is AA's HQ, LHR is one of AA's most 'premium' markets and the 77W is AA's flagship aircraft - I don't see the 77W being moved to another route at the expense of this one.