Originally Posted by
SFOPhD
Most answers here:
http://www.aa.com/i18n/utility/sameDayTravel.jsp
In short, a same-day flight change can be made within 24 hours of the flight if the new flight:
"Has the same origin AND destination
Is the same calendar day of departure
Has eligible seats available"
FWIW, the routing that you take to get to the destination from the origin can be different in my experience. I have done this from Seattle, for example, where I was on a red-eye SEA -> DFW -> ORD and changed to earlier in the day SEA -> ORD.