I just experienced this on the phone right now (I am US-based and purchased on singaporeair.com).

I have two Flexi Saver tickets that I want to move the date by one day and the rep on the phone wanted $50 each for the date change (unexpected), plus fare difference (expected). He stated that it was not a rebooking fee, but a "telephone convenience fee" because the SQ website cannot handle rebooking when there is a fare difference.