I believe that usually you have to cancel the original booking and then see if the seats in First are still available to book on the flight you want using your Avios and voucher and paying the extra taxes. The risk is that by cancelling you may lose the seats altogether if the seats in First have disappeared. You should be able to do this online, but might be worth you calling BA in this instance. You have to cancel before 24hrs of the outward flight though. Good luck.