If it makes you feel any better please know that you aren't missing out on much going from economy to business since you are travelling on a narrow body 737. At the end of the day you'll have access to your First class lounge as a Qantas World Platinum, you'll have First class check-in boarding, etc. Yes the seat is a bit narrower than the business class but is it worth paying through the nose for an upgrade to business? Highly doubt it! And I mean look, even if you got a discount economy fare that was upgradable you are looking at 21,800 points just for the upgrade each way (i.e. 43,600 points). For fewer points than that you can book a classic business award from SYD to PER in J on one of their A330s which actually has a lie flat business class seat and service!
One final data point - I was travelling on a very pricey Economy Flex ticket with Qantas last year from SYD to CHC to attend a conference. The cost to upgrade was 11,800 points and I thought what the heck may as well request the upgrade. I did so and the upgrade didn't clear for either the outbound or inbound flight. This despite me being a QFF Gold and travelling on a fairly pricey fare. So even if you could make the change the odds of it actually clearing isn't a given, particularly on a narrow body where the number of J seats by definition will be limited.
-RooFlyer88