Hi

I have previously flown AA J class and on the day of departure, at on-line check-in was able to chose an earlier flight. This was a MIA-JFK-LHR itinerary, and was able to take an earlier MIA-JFK flight. Is this option only available for all AA J class flights or just those departing USA airports?
BTW, my ticket was the cheapest business class fare and not a fully flexible ticket

Thanks