Yes, for Upgrade using Avios you can simply cancel the UuA, get your Avios back, and most of the taxes, you do lose a redeposit charge of £35 in your account is based in the UK, no charge if it's a Gold Priority Reward. Your ticket then reverts back to its previous status. The deadline for doing this is 24 hours before the UuA flight.

It is not the same with POUGs (and effectively AUPs), cash upgrades, where the ticket loses its flexibility on the leg concerned.