All changes must be made before the originally booked date of travel. If you miss your flight, it may not be possible to rebook using your current ticket even if you have purchased a MultiFLEX pass.
No shows may also have an effect on remaining segments of your tickets, possibly resulting in a new purchase being required.
As such, please try and make all changes as far in advance as of your original/proposed travel date (whichever is sooner) as possible to ensure good availability.
If you change your ticket to another travel season, or at the last minute then you will be responsible for any difference in flight price.