Originally Posted by jo.van

Last time it was early this year for flight in September - there was a minor change of 30 minutes later for SIN-IST segment, and it allowed me to change previous leg SYD-IST to a different flight on the same day (I could also select other days).

IIRC, you need to click on Accept button on SQ web site (nerve-wracking, I know ), and then it will let you change flights before finally confirming.