Originally Posted by Jyrsa

Do you mean you paid the taxes but received a return of the taxes later? It's sometimes possible that a booking isn't ticketed correctly. Have you received the actual ticket via email? If not, you should contact CS

You can only upgrade an award booking by vouchers or cash at the airport. Not avios, as this would usually result in a lower avios value than booking business.

You can also cancel the original award ticket for an almost full refund and book a business ticket instead. There is a 50€ cancellation fee for award tix that at least in the past has been waived for plats and Lumos.