Originally Posted by Dave Noble

Given that you have only part paid the booking, has the ticket actually been issued yet?

If there has not been a ticket issued , then there will be no reimbursement due - in this case you should talk to BA holidays about repricing due to the change in cabin

If the ticket has been issued, then reimbursement would only become due after the flight has been taken, so would not be deducted from the amount still owing