Yes, unfortunately upgrade space is not guaranteed. The safest route is to check for availability of award tickets in the class you want to travel prior to booking a lower class, but that doesn't perfectly guarantee upgrade availability either. There is no way I know of to tell upgrade availability with 100% reliability prior to booking. And SQ will let the plane fly with empty seats in Business even if people want to upgrade, because otherwise more people would buy PE hoping for an upgrade, whereas SQ wants you to buy Business outright.