Sounds like a tail swap from 16 seat F to 12 seat F. In the past when they first started, AS was offering the difference between the purchase price and lowest coach plus travel certificates but they seem to be less generous as time went on.
If it were me, I'd check pricing on other carriers first to see if canceling and rebooking would pencil out. If not, ask for cash for the downgrade difference plus a single travel certificate for the inconvenience and use that certificate to purchase the third seat.