When I went to make some routing changes on a DONE5 with the AA RTW desk, the initial change quote was exceedingly high (something like +$600 per ticket). I then called back with a changed routing request, and the change quote was now $42, which included the $125 change fee (i.e. the new itinerary resulted in lower fees/taxes than the original booking, and these were apparently refunded to me in the net calculation).

N.B. Routing changes that include new stopovers in the U.K are very, very expensive....