Originally Posted by demue

I'd not do that, very risky. SQ has been taking a hard stance on reinstating expired miles. This may be confusing, but they distinguish between expired miles in the account and as part of a booking

If the miles are in the account and about to expire, SQ has been extending them voluntarily. However, if the expired miles were / are part of an existing booking and you cancel, those miles will not be credited back / not be reinstated. E.g. they will not credit them to the KF account and then automatically apply the same extension rules as are used for miles that were sitting in the KF account. A weird and unfavorable distinction, but very important to be aware of or lose the miles.

Best action for OP is to suck it up, call in (yes, long hold times) and rebook the same or different routing in the future. (Beware that if new routing requires less miles, you don't get those back either, but that is based on a different existing KF rule). The reason OP can't do the change online, very likely, is that the max ticket validity is reached (365 days from original issue date) and to go beyond that it needs an agent to reissue the ticket.