This is what I found-
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I have confirmed and ticketed my redemption booking but I would like to cancel it now. How should I go about it?
If the ticket for your redemption booking has been issued, but it has not been used, please call KrisFlyer Membership Services to cancel the booking. Miles will only be re-deposited into your KrisFlyer account if they are still valid and have not expired. A service fee will apply, and is chargeable at 5,000 KrisFlyer miles / USD50 for KrisFlyer members, or 2,500 miles / USD25 for KrisFlyer Elite and PPS Club members.
If the cancellation of ticket is within 24 hours of the scheduled flight departure, a cancellation fee of 7,500 miles / USD75 will also apply, in addition to the service fee above."
If you book online, I think you can cancel online also and fee is 3,000 KF miles or $30USD , according to service fee schedule (figures are pts/dollars for KF members, KF silver and KFgold/PPS/Solitaire):
Redeposit miles of completely unused tickets (subject to validity of miles) online 3,000 30 1,500 15 1,500 15
Redeposit miles of completely unused tickets (subject to validity of miles) offline 5,000 50 2,500 25 2,500 25