This is now a serious problem with Krisflyer which I have encountered twice in recent months, where agents insist that award availability which can be seen on the website can only be used for NEW award bookings and is not available for use to rebook EXISTING award bookings. And just to be clear, its nothing to do with Saver v Advantage bookings. In both cases, I was trying to change an Advantage booking (which should be freely changeable) to a later flight which (as the agents confirmed) had plenty of award seats available in Advantage. But in both cases the agents (one of whom was very helpful and apologetic and told me other people had encountered this before) insisted these award seats were only available for NEW award bookings.

Although the agents never suggested anything of the kind, its just possible that this was part of some weird implementation of married segments since in both cases I was trying to change the HKG-SIN segment (where there was ample award availability) of a HKG-SIN-PER booking where the SIN-PER segment (which was on a different day, and I was not trying to change in any way) no longer had any award availability. Maybe Krisflyer doesn't allow you to change one segment of a bookiing unless there is still award availability in all segments of that booking (even though you're not making any changes to the other segments)?

In any case, definitely something to be aware of if you're making an award booking (especially with more than one segment) which you think you might later want to change.