Connecting tickets on BA don't have a standby or SDC facility, that's a USA/Canada trait not much seen elsewhere. You can ask at DUB if it's ok to go on the earlier service, but normally the reply would be "sorry we can't do that". Occasionally they will say yes, either because of a weather advisory or due to a known overbooking issue later on. Award tickets issued by another airline is about the hardest thing to change too, if it's a big deal you best call AA and see what they can offer.