The problem is that AA insists on having the ticket reissued for a date change, and this is then in coach not business in the coupon. Trying to use the originally issued coupon is not allowed and AA will cancel the reservation if you don't reissue the award, at least in theory. Not sure if forgetting to get the ticket reissued and showing up for the flight will work or not (obviously this only applies to non-eticket awards, and chances are that is an eticket, in which case the reissue was done when the PNR was changed).