Changing a return ticket for low-cost air flights (ex. Wizz Air) requires paying half of the original price. However, if you have previously paid for a certain amount of luggage, do you have to pay for it again?
From the information provided online it seems that you may only have to pay for a date change which is as follows:
Flight change fee
50% of the original fare. Minimum and maximum amount of the fee is shown in the table.
via wizzair.com Per flight, per passenger €10.00 - €60.00 / £9.00 - £53.00
This accounts for the original fare only. The baggage which you purchased is a separate add-on just like WIZZ Xpress priority boarding etc; so they won't account to date changes. If you are still not satisfied you should contact there customer service @ 0330 977 0444 (UK Number)