Arrived early at BKK for a DONE5 segment to HKG on CX. There was an earlier flight with D availability which I could easily make but the check-in agent refused to even look at changing it saying I had to contact BA (their desk was closed) if I wanted to change anything - I then had to wait for my original flight.

Was she correct or should I have pushed to see a supervisor - I had assumed that any Oneworld carrier can make changes on the day of flying?

Thanks