In theory BA is meant to be able to do it for you, with a service fee.

If the ticket is issued by AA however, they should have "full control" over all segments of the ticket regardless of what GDS the operating carrier/s of the individual segments are hosted on. I'm suspecting that you got a newbie (a lot of them around these days) at the ATW desk as IME they've always been more than happy to help me alter my itinerary on AA-issued tickets, regardless of operating carrier.