If you miss your BA flight (booked as a partner award with AA miles), you'll be able to rebook to a later flight IF BA has seats on the later flight. However, you'll have to make the change through AA, not BA, so be sure you've got the number with you to reach AA from outside the US.

If the later BA flights are full you'll have a little bit more of a situation, if your all-partner award includes the connection then you're just subject to normal rules governing misconnects, which is to say AA will get you to your destination but it may not be on the next flight.

You might consider moving to the later flight now if the inventory is there just to put your mind at rest.