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"
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.
Not entirely true.
The itinerary change can be done before or after commencing travel and can involve a change in origin/destination. For example a wholly unused ticket from DFW-ATL RT for next month could be used toward the purchase of a new ticket for next December to fly SEA-CHI RT.
Any differene in fare would apply. In addition, if you have already commenced travel, you must satisfy all of the provisions of the used fare portion or an ad-collect to applicable fare would occur (i.e. if no longer staying a Saturday night and the outbound used a fare that required it, you would be ad-collected up to the fare amount that would have applied to your trip as flown.
You just don't pay a change fee.