Originally Posted by KingCanute

If you cancel the return before flying the outbound, the ticket will be repriced at the lowest available one way fare on the day you make the change. Changing the date of the return before flying the outbound may also result in the whole ticket being repriced at that day's prices. There may also be a change or cancellation penalty to pay, which could be whatever the most restrictive fare rule states for the penalties.

If you amend the return after flying the outbound, it should be repriced at the rates available on the day you made the booking. But your cheap fare bucket may not be available anymore by then so you might have to pay a hefty fare difference, plus any change penalty.

If you cancel the return after flying the outbound, QR may notionally reprice the outbound as a one way and reduce the refund accordingly, and again subject to the penalties in the fare rules.

There's not really any way for one direction not to be affected in some way by changes to the other, unless you buy separate one way fares. This applies to redemptions as well so it is often more flexible to book those as one ways and is no different in price. Changes to redemption tickets are however subject to there being availability for new redemption tickets on the day you make the change, which can be tricky.

So no clear answer for you there I'm afraid. Whether you book now or later there's some degree of gamble involved if your dates are likely to change