I've actually done this. I had a flight booked months ago, which changed flight times 3 times over, first by a small amount, but it ended up about 5 hours different. The new time actually suits me, but that's by the by. I was sent the same email on the first move, by the time I decided to summon up the energy to look at it, the fare had also dropped by about 20 Euros. So I cancelled the original and asked for the 20% voucher. At the last stage of this process it said it may take 48 hours to be added to my account, but in reality it took under 5 minutes. After I logged in then the voucher value was there, I rebooked, leaving a residual. I then moved seats for an extra 2€ after a large party moved up to my seat, and was able to draw down on the residual more easily than using a credit card. Clearly you need to have a profile with Wizz but it's clear and easy to use.