Section I (13) of T&C:
KrisFlyer miles of completely unused award tickets may be redeposited for a fee provided the KrisFlyer miles and the ticket are valid at the time of redepositing. Any taxes will be refunded. KrisFlyer miles cannot be refunded for partially used award tickets and upgrades.
If miles would not have been valid when you cancel then you won’t get them back. Time and date changes are fine though, within validity of ticket.