When changing the date of a flight, it is important that you take the ticket difference into mind. In the event that the new flight is more expensive than the original, passengers might have to pay the difference even if the airline charges an affordable change fee. If the new flight is less expensive, however, some airlines might give you a credit or voucher for a future trip. Travellers may be able to estimate the overall cost of changing their flight date by staying familiar with pricing changes.
(https://airlines-office.com/blog/how-to-change-flight-date-for-free/)
