I agree with the above but the key - so I have found - is to insist the agent you are speaking to deletes the existing seat assignment and then new ones can be made (and this has to be done with an IB call agent - BA call centre cannot do it even if BA code share).

Asking them to "change" the seat assignment will result in the agent not succeeding - delete seat assignment and then assign new is how to do it.

BTW, the online chat facility can be used to do the above if you get the right agent who also is willing to take the above instructions