Yes, but that is not what the OP wants. The OP wants to resolve the schedule change on CPH-LHR or LHR-SEA (btw you do not say which changed nor what were your original timings, but given what you describe I imagine that you are currently to be on the Thursday 8.50 from LHR, arriving at 8.25 from CPH?) by a change of route: CPH-LHR, LHR-LAX. The OP is correct that BA will most certainly refuse to create award space on LHR-LAX, which sells extremely well in F in the first place, where it can instead offer a later LHR-SEA.

BA may also be amenable to letting you fly on another day (on the same route) if it is of interest as the SEA flight only goes at 8.50 one day a week.

Unfortunately, both airlines tend to be rather narrow minded when one seeks to resolve a timetable change through a rerouting on a different origin or destination, but if you want to go that way, I would suggest pushing the AA flight option as it is (in my view) slightly less unlikely that they will say yes compared to BA.