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"

An itinerary change can be made any time before flying and still must have the same origin and destination but the new flight does not have to be on the same calendar day of departure.