The joys of oneworld.....

I'm currently traveling on a AA issued DONE5 (paper ticket) and tried to change the date of one of my BA flights.

So I called BA, but they refused to change my reservation because it was issued by a travel agent? (and they couldn't lookup the fare rules)

AA told me to call BA, because it was their flight, and they use a different reservation system...

So now I will have to no-show a flight, which BA probably won't like. (I don't care, as most of my flights are open dated, so canceling my reservations isn't going to impact me.)

Are there any rules about which airline is responsible for date changes? I always thought it was the operating carrier?