Originally Posted by
OC37
"If your flight is cancelled, you are entitled to a full refund, as per existing British Airways policies."
But it seems that the flight hasn't been cancelled, only a change of aircraft type.
Apparently the OP simply booked the flight and chose seats, presumably online, without informing BA of any special requirements. In his place that's probably what I'd have done – I probably wouldn't have thought of the contingency of change of aircraft. But I think that if the OP had told BA at booking time that he required a particular type of seat (aisle), giving reasons – medical or any other – then that requirement would have become part of his contract with BA (there's a legal phrase which I forget) and when the aircraft type changed, BA would have been obliged to provide equivalent seats. That's my understanding of UK law – I gather that the OP was in Spain, and I don't know what effect that would have had.