Confirmed Flight Change (AA)
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Same Day Flight Change (f.k.a. Confirmed Flight Change)
American Airlines allows changing of a confirmed reservation for (a) flight(s) ticketed on a "domestic" (U.S., Puerto Rico, U.S. Virgin Islands, Canada and the Caribbean) fare for a fee. More about the program on AA.com.
The information on AA.com is incomplete, and the complete information is part of the rules of the fare that you purchased. Because AA.com does not allow you to display rules pertaining to tickets you have already purchased, below is the text from a typical rule for fares purchased on 24FEB13 as an example.
05.ADVANCE RESERVATIONS/TICKETING [...] 2. SAME DAY FLIGHT CHANGE/SDFC/ A. ELIGIBILITY 1.PASSENGERS WHO PURCHASED TICKETS FOR FIRST OR BUSINESS OR COACH CLASS BOOKED IN Y/D/I/J/A/P/F INVENTORIES MAY USE SAME DAY FLIGHT CHANGE WITHOUT CHARGE AS NOTED BELOW. 2.PASSENGERS TRAVELING WITHIN THE US48 WITH FARE BASIS CODES ENDING IN 7 OR 8 OR 9 MAY USE SAME DAY FLIGHT CHANGE FREE OF CHARGE AS NOTED BELOW. 3.ALL OTHER PASSENGERS PURCHASING COACH FARE TICKETS BOOKED IN H/B/K/M/L/W/V/G/S/N/Q/O INVENTORIES MAY CONFIRM TRAVEL USING SAME DAY FLIGHT CHANGE FOR A NONREFUNDABLE FEE /SEE AA.COM FOR DETAILS/. ///---///---///---///---///---///---///---///---// B.SAME DAY FLIGHT CHANGE CONDITIONS FOR ELIG PSGRS 1.THE REQUEST FOR SDFC IS MADE WITHIN 12 HOURS OF THE ALTERNATE FLIGHT/S DEPARTURE TIME. 2.THERE IS NO CHANGE TO THE ORIGIN/DESTINATION OR STOPOVER POINTS. 3.NEW FLIGHT/S/ AND ROUTING ARE PERMITTED PER THE TICKETED FARE RULE. 4.ALL TRAVEL IS ON AMERICAN AIRLINES/ AMERICAN EAGLE AND AMERICAN CONNECTION. 5.REQUESTS ARE MADE VIA AIRPORT SELF-SERVICE CHECK-IN/ TICKET COUNTERS /INCLUDING ADMIRALS CLUBS AND PREMIUM SERVICES/ OR BY CALLING AA RESERVATIONS. 6.CHANGES PERMITTED TO OUTBOUND OR CONTINUING/ RETURN FLIGHTS. 7.CONFIRMATION IS SUBJECT TO AVAILABILITY. 8.ALTERNATE FLIGHT IS ON SAME TICKETED DATE. ///--///--///--///--///--///--///--///--///--/// REQUESTS MADE MORE THAN 12 HOURS IN ADVANCE OF THE ALTERNATE FLIGHT/S DEPARTURE TIME WILL BE PROCESSED FOLLOWING THE GUIDELINES FOR RESERVATION CHANGES/ WHICH MAY INCLUDE COLLECTION OF APPLICABLE CHANGE FEES PLUS DIFFERENCE IN FARE. SEE PENALTIES
A Same Day Flight Change will be granted if there is E inventory available; otherwise it will be denied. You can proactively check E inventory by calling AA or a Travel Agent; there is no way to do so online for free, but some paid services will show it.
You will be rebooked in the same inventory you were ticketed in, even if sold out, as long as, and only if, there's E inventory available; if E inventory is sold out, a Same Day Flight Change will not be granted even if the inventory you were ticketed in is available.
Alternatively, you can go standby (unconfirmed): see Standby (AA)