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[edit] 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 05OCT09 as an example.

16.PENALTIES
   FARE RULE 
   CHANGES
[...]
        V. SAME DAY - CONFIRMED FLIGHT CHANGE/CFC/-           
           PASSENGERS HOLDING CONFIRMED RESERVATIONS AND     
           TICKETS IN INVENTORIES K/M/L/W/V/G/S/N/Q/O MAY    
           CONFIRM TRAVEL ON AN ALTERNATE FLIGHT FOR A        
           NONREFUNDABLE 50.00USD FEE WITHIN 12 HOURS OF      
           THE ALTERNATE FLIGHT/S DEPARTURE TIME BASED ON     
           THE FOLLOWING CONDITIONS                           
        ---                                                   
          A.THERE IS NO CHANGE TO THE ORIGIN/DESTINATION      
            OR STOPOVER POINTS.                               
        ---                                                   
          B.NEW FLIGHT/S/ AND ROUTING ARE PERMITTED PER       
            THE TICKETED FARE RULE.                           
        ---                                                   
          C.ALL TRAVEL IS ON AMERICAN AIRLINES/ AMERICAN      
            EAGLE AND AMERICAN CONNECTION.                    
        ---                                                   
          D.REQUESTS ARE MADE VIA AIRPORT SELF-SERVICE        
            CHECK-IN/ TICKET COUNTERS /INCLUDING ADMIRALS     
            CLUBS AND PREMIUM SERVICES/ OR BY CALLING AA      
            RESERVATIONS.                                    
        ---                                                  
          E.CHANGES PERMITTED TO OUTBOUND OR CONTINUING/      
            RETURN FLIGHTS.                                   
        ---                                                   
          F.CONFIRMATION IS SUBJECT TO AVAILABILITY.          
        ---                                                   
          G.ALTERNATE FLIGHT IS ON SAME TICKETED DATE.        
        ///--///--///--///--///--///--///--///--///--///      
        PASSENGERS TICKETED IN Y/B/H INVENTORIES MAY          
        CONFIRM TRAVEL WITHIN 12 HOURS OF THE ALTERNATE       
        FLIGHT/S DEPARTURE TIME FREE OF CHARGE FOLLOWING      
        THE ABOVE CONDITIONS.                                 
        ///--///--///--///--///--///--///--///--///--///      

Alternatively, you can go standby (unconfirmed) for free: see Standby (AA)