The lack of tax calc makes it impossible for AA to re-issue this ticket -- their system will not let them, and no supervisor can override this. Also AA doesn't use stickers like BA does (they reprint the flight coupons involved), so you will find life is much (MUCH) easier if you get BA to sticker your 2 flight coupons with the DFW in A change; in my experience they will do this readily and not charge for a ticket re-issue (AA is almost sure to charge you USD75 for this). Doing it in the UK before you leave will be the best strategy.
While I agree that the AA RTW desk is excellent, they are not for this circumstance and you aren't needing any help with selecting flights. The weak point of AA is their ticketing process for RTW tickets is a nightmare (have to hand-write the tickets and 99% of the AA agents cannot do this, it is a fine art to get a reissue done properly).