Originally Posted by marcopizzaiuolo

are you sure, on the AA forum there was a full agreement that OWE could select MainCabin
Extra seats.

AA has 24hrs free cancellation, so one way to be sure for the OP would be to book Basic Economy, add BAEC number to the booking and select seat, to confirm whereby Main Cabin Extra can be selected. If not, the booking can be cancelled within the first 24hrs and fully refunded, to book again Main Cabin (where I am sure, based on first hand experience, that Main cabin Extra can be selected).

all the other bene first (priority boarding, luggage allowance, lounge access, are definitive).