I booked a flight from Buenos Aires to London through Iberia (one of the legs is via British Airways). When I get to fill my personal information, the frequent flyer number is already filled with my British Airways number (on all the flights for both airlines) and I can't change it... Does anybody know if there a way to change it (to AA) online without having to ring customer service?
If you booked the flight with British Airways or Iberia (or another airline who uses the Amadeus computer system), you can use the PNR (the alphanumeric reference number you were given) to examine your booking via the websites of Finnair, Royal Jordanian or Qatar Airways.
The reason is that, their websites are just a front for the underlying Amadeus computer system (that BA also uses). Those three carriers have decided to expose more functionality through the website: any of those three will let you change the frequent flier number.
Qatarairways.com is probably the least "buggy" for this exact purpose (believe it or not).