I once had to do it. I was told that paid upgrades are non-refundable in case of voluntary changes/cancellations. However, it will be refunded if you cancel paid upgrade and book a new upgrade. I purchased upgrade to A on LX for 1,300 CHF and then had to change the flight. On the day of the new travel, A class was not available, so I could have refunded A class paid upgrade only if I later purchased new available paid upgrade (which was almost 3,500 CHF because only F was available). I found a date when A was available and could purchase the upgrade at the same price as the original one (and the original one was refunded). I was told that the original EMD cannot be applied to the new upgrade, even if the price is the same.