It is an AA flight. So I should call the Plat desk and ask them to book seats on the new flight under a new PNR without touching my existing PNR (which they say they can't touch)? And then call RTW tomorrow to sort the existing PNR out?

When I called the Plat desk earlier, as soon as I mentioned Oneworld Explorer the agent said I would need to call the RTW desk when they open. How can I get around this (I imagine I need to mention the fare code for them to know to book the seats in D)?