Consider this scenario: You bought and paid for a first-class or business-class airplane seat. You have a boarding pass, but when you arrive at the departure gate, the agent tells you your seat has disappeared.

Either the airline changed the equipment, and the replacement aircraft is configured with fewer seats, or the seat you had is broken. Or — and this has happened a lot lately — an off-duty pilot is now flying as a passenger and has priority. Or, your earlier flight on the same airline was delayed, and the missed connection because of that delay has now put you in a coach seat on your onward flight. Either way, you’ve been downgraded a class or two.

Can the airline do that? What are your rights? And are you entitled to a refund of the fare difference between what you paid to sit in the front of the plane and the fare category to which you’ve been moved?