New option at Wizz Air, WIZZ Flex. Purchased at the time of booking, it will allow pax to change the date, time and routing of the flight, online and any number of times, up to 3 hours prior to the scheduled departure time. The change fee will not be charged, however any fare difference must be paid.
If unsure about the day of departure... why not? It costs just 10 EUR / 9 GBP versus 30 EUR / 26 GBP if changed with no Flex option.