Originally Posted by
izzik
You can also just skip adding the FF#, then request credit for individual flights with each program after travel is completed.
Obviously, the downside is that if there are benefits tied to having the program on the ticket, you may not be able to use those (eg, seat assignments). YMMV
American explicitly
prohibits changing the FFN if you obtain any benefits (more of issue in coach bookings) and also disallows number change post flight:
(4) You cannot accrue partner airline miles with your partner frequent flyer number if you have already used your AAdvantage®
number to obtain any AAdvantage®
Rewards and Benefits such as First or Business Class upgrades, baggage fee waivers, priority boarding, access to Preferred seats or complimentary access to Preferred / Main Cabin Extra seats
(7) After your flight departs, you will not be able to change your selection of a frequent traveler program with respect to that flight.
In the past I used FFN status to access first class CX lounge at LHR and also BA LHR First lounge just by showing my status card when the ticket had a different FFN but not sure if that works in other lounges.