I would take the word "cancel" with a pinch of salt. For any of the changes which you're contemplating, you're not really so much "cancelling" as "changing" your booking, even if you technically change the routing (LHR-MEL on a direct flight that stops at SIN can be technically regarded as a different routing from LHR-SIN followed by SIN-MEL).

What you can do and how much it would cost will depend on your original fare rules and what availability there is in which booking classes on the flights you want to change to. I think that, to be realistic, there's no real substitute for phoning to ask for an indication of the likely cost when you have an idea of which flights you might want to change to.I would not do this for this much money per person. It just isn't worth the money (just as $1800 per person to upgrade from economy to premium economy is also absolutely not worth the money).