If the aircraft changed that resulted in fewer F seats (e.g. A320 to A319), and you were seated in one of the rows that no longer exist, that could also happen. Lesson there is, if possible, always book in row 1 or 2 to increase your survival chances on an aircraft downgrade.
What happens if you're swapped from a plane with rows 1-5 to a plane with rows 3-6
