Thanks, Dilly. I had called BA again before I saw your post, and this time my request for a seating change went through, so perhaps the problem on the first call was that the agent didn't make the change properly. Of course that doesn't explain why all my efforts to change the seat in MMB failed, but I gather that's a more widespread problem. Nor does it explain why a window seat on the 747 UD and a recorded preference for aisle seats morphed into a middle seat on the conversion to a 777 (maybe an effort to reconcile the two?)
In any event, I really appreciate your offer to help, particularly as I've been finding BA generally unresponsive on this round.