Standby is a North American thing. Normally there is no standby on European airlines (other than where flexibility is built into the ticket, certain staff tickets or possibly in irrops situations) and you are required to fly on the original flight as booked unless you want a full re-fare at walk-up price. BA has free sameday change fares but you won't be on one of these if you are on an award. You can always try your luck asking at the gate but I would have thought that your chances of success are close to zero. It might work if you can get AA to change your award but I think that it would probably be too late to do that by the time you land in AMS