The SQ revenue management system sucks. What happen is they hold inventory for sales only but not allows to rebook. It is really not a matter of how long of the wait list or how much time required to clear a booking.
Here is my personal experience:
I booked a flexible ticket on "H" class from Singapore to Sydney with the cost of $x. The ticket is free for day and flight change.
Due to change of plan, I decided to re-book the inbound flight on 01 Jan where I found the same price ticket $x is available for sale on SQ.com on the 01 Jan. However, when I call the SQ reservation, they keep saying there is not "H" class available, and only "B" class as minimum. The only solution they have is to get 8th Jan asthe earlier "H" class available after 1 Jan or pay extra S$200 to upgrade one way from "H" to "B". They said ticket available for sales does not mean you can get that with your issued ticket! Obviously, they restrict your accessability to flight once your ticket has been issued. Thats is very doggy and they always try not to get you a seat unless you are yet to pay..........
Anyone of you have a issued ticket may try, you will see how ridiculous is this airline and I shall NEVER fly again. Sucker!