Definitas, in the event of a cabin closure your choice is between accepting the lower cabin on the same flight or rebooking on a new date (-/+ 3 days from the original date only in case the original booking class is unavailable, otherwise, you can choose any date within the ticket validity). You will be able to amend the return flight to give you the same length of the trip. Rebooking on AA is not an option, nor is getting a refund.

I missed that it was an Avios booking, so, of course, a refund is possible minus the fee.