Originally Posted by mygirlsmom

You can call AA to change your flight. Z inventory must be available and AA will reissue your ticket. The only hitch I see is that sometimes, when the check in process has already begun, AA won't be able to reissue over the phone, which means going to the AA counter at LHR to reissue in person, which of course is time consuming. I would definately try, because the agent will be able to tell you before hanging up if they will be able to reissue over the phone.