Originally Posted by
KARFA
It seems you are talking about doing changes on the day in the lounge. This is after control of the booking has passed from BA to CX - you are free to negotiate for whatever you want directly with CX, and CX are able to change the booking themselves at that stage.
The OP's example (and what I am talking about) is a change some weeks before, not "on the day" changes. For all reward bookings you have to sort it out with BA at that stage and the operating carrier CX/AA/JL etc wouldn't be able to help.
This makes perfect sense. What I meant to say above is that CX could have moved a passenger to another flight on the day of the flight. Not weeks ahead.