I've not yet tried to get AA RTW desk to take control of the ticket but once I've sorted the current changes out I will definitely be looking into it ....
And I did ask BA agents what would the situation be if I wanted to make the change to a flight on the same day rather than in a week's time and they said -well we would have to try and sort it out ...
I went to the ticketing desks in Terminal 5 over by the First Check In area and the agent there was v helpful and did get the rerouting set up in the system but the compuer wouldn't compute and process the taxes ec
So I woud certinly give it a go when you land ...
And I'll update in due course - a v helpful agent at Exec club has given me his extension and assures me that he will make sure somebody sorts if out for me first thing in the morning ...