As a reference point, I have a LGW-SIN-CGK booked on a cheap economy fare. They messaged me that the LGW departure time had been brought forward 10 minutes. Not a big change but my schedule to get to Gatwick is already going to be tight so out of caution I asked whether they could change me instead to the next flight an hour or so later, from Heath Row not Gatwick (keeping the original SIN-CGK leg intact). When I finally navigated their banal automated menu to speak to a human, SQ's phone agent said no as the change was under an hour and I was on an economy light ticket. As a passenger I'm not sure the ticket fare bucket should be relevant, if the logic is that a time change on the airline's part means the flight no longer works for the passenger's schedule.
In my case, not a big deal. But good to have a data point.