You don't need to change your ticket - you only need to change your reservation. xONEn ticket coupons are not endorsed "valid flt date only" so they can be used on any flight on that route. If the flight looks like it might be full then you can ask CX for a reservation in the appropriate class to hold a seat, for which they won't charge. But you can also just turn up and go if there is a seat available.

This is all much easier with paper tickets - one of a number of reasons why I still prefer them.