When you change a ticket, the new ticket must meet all the conditions of the original fare, including availability. Accordingly, if you change a ticket to a date on which the appropriate fare is more expensive (e.g. from a weekday to a weekend) you are responsible for not only the change fee, but also the fare difference.

In the case of a reward ticket, you cannot meet the conditions of the original tarriff, because the original tariff is blacked out for the date to which you want to change.

C'est daumage.