Unless you are near a city ticket office for one of the ow carriers (doubtful) then the airport is pretty much the only way to do it. If you don't want to wait for hours call the AA RTW desk first to set up the changes (1-800-247-3247). That way all the agent has to do at the airport is check the original ticket, print off the new one and collect the money (which should still be $75 - not $100).