BA Policy
Paid for seats are refundable when BA cancel your flight and that happened automatically to me last September. Fortunately the credit card used to pay for them was still valid because they didn't ask or tell me what was done.
Putting me in economy seats for a two leg replacement journey instead of the business seats on the cancelled direct flight did not generate a refund of the difference between business and economy. Apparently it is not BA policy to refund in this situation because 'we got you there, didn't we?'
I'm glad I didn't pay for first class seats!