Is this a Web Special? I just spent over an hour on the phone with AA, trying to rebook a Web Special J award I had booked to BUD (42k miles).
After talking to multiple supervisors, the agent came back and said that there was literally nothing she could do - apparently they aren't allowed to change the origin or destination at all for Web Specials, nor rebook on another carrier even if there's award space (BA had availability, but would have required booking an entirely new award at 57.5k + $700...no thanks). According to her, they "might have been able to do something" if it was a standard award.

She did say that a report had been filed to consider a more flexible policy for situations like this, but advised me not to expect a resolution anytime soon.

I might try calling again later this week, but will probably end up booking a new Web Special to LHR and catching a paid flight on BA from there.