Originally Posted by
OldLurker
... 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..
I think the legal phrase is that the seat requirement was "of the essence" to the contract for the flight tickets. He would have had to say that before buying the tickets - and BA might have said that they couldn't accept that, and so refused to enter into a contract with him.
If there's a better phrase, I'm sure someone will correct me...