Who to contact in regards to changing British Airways redemption flights 15th Dec 2024?

We are currently flying in BA Club World LHR to SEA but would like to change this flight to LHR to YVR. I can see that there are more unallocated seats for the Vancouver flight, and seems like a logical thing to allow. As there are no redemption seats any BA representative I talk to is unable to make any changes.

Keen to make the change to avoid the extra car journey and customs, half wondering about changing on the day of the flight, but would much prefer making the change as soon as possible. Seems though that I am not able to find the right person/team to ask, Any advice?