I changed an LGW-DUB ticket booked on americanairlines.co.uk by calling them in USA (it was 0300 and needed to change ticket urgently). They told me that I coulnd't take the extra fee and that I would be able to pay at the airport. Sure enough, at check in I was directed to ticket desk to pay the extra fee... which turned out to be about £20 more than AA had quoted!