You may find the Frequent Flyer Number area grayed out, or be unable to change FFN online via aa.com. Alternatives:

Call AA reservations at 800-433-7300.

Tweet @AmericanAir and ask the Twitter team to do it - though the best way is to have them follow you first and then send them a DM to keep your information private.

Request the FFN change at the check-in counter or, if you arrive before they get very busy, the gate.

Using the Finnair or Royal Jordanian website to change your FFN seems to no longer work. (AA use Sabre, several other one world airlines use Amadeus.)

NOTE: If switching from AAdvantage to another scheme's FFN, please be aware: Benefitting from a privilege granted for your AAdvantage status and changing your FFN to earn on another airline is generally prohibited by AAdvantage Terms and Conditions. Doing so might risk that benefit (e.g. upgrade).

From the AAdvantage Terms and Conditions (link)

Earning AAdvantage Miles

"You cannot accrue partner airline miles with your partner frequent flyer number if you have already used your AAdvantage number to obtain AAdvantage program benefits such as First or Business Class upgrades, baggage fee waivers or complimentary access to Preferred/Main Cabin Extra seats. Additionally, you cannot redeem partner airline miles with your partner frequent flyer number and obtain AAdvantage program benefits such as priority boarding and access to preferred seats."

Older and obsolete posts have been moved to ARCHIVE: Changing FFN / Frequent Flyer Number on AA Bookings

Updated 5 Jul 2017 by JDiver