ba - seat change
I had the same thing happen to me last year. booked and paid for a specific seat on long haul flight. Aircraft type was changed after original booking but called BA and they rebooked me onto a seat in roughly the same area.
Checked in online for both sectors of my flight (with my original seat number given), arrived at the check in desk at MAN to be told seat on the long haul sector had been changed - due to aircraft change!.
Arrived LHR and double checked, told the aircraft was the same type but configured differently and that the seat i had booked did not exist on the aircraft. Very unhappy about it, but accepted the inevitable. Imagine when i got on the aircraft to find that the very seat I had booked DID exist and that someone else was sat in it - and I had paid for the priviledge of them sitting there.
I have to say the CC was fantastic though and moved the person from the seat - but still no answers from BA as to why they did it!
Needless to say will be avoiding them for my long haul flight next year.