As stated above, I redeemed a flight through Asia miles in J (so Cathay is the travel agent). I called them to rebook and the agent was entirely unhelpful and kept repeating that there were no other available rewards flights and didn't quite understand the situation/that there is an option to switch. In this instance, do I call BA directly to have them force open award seats? How else should I go about getting CX in touch with BA?

Thank you all, this thread has been extremely helpful.