I’d look at the cost of upgrading current flights first. You get a better seat, service etc and more status points and it doesn’t take up any more of your time.
Otherwise you could post some dates to fly and your departure airport either here or on the Flyertalk Air New Zealand forum. I mention that because it sees a lot more traffic so you’re more likely to get faster responses drawing on a lot of different peoples experiences as it’s not an uncommon question.
There’s no booking change fee for upgrading flights to the islands or across the ditch. Pay the obvious fare difference. You can change it online or by phoning. Phoning incurs a service fee – maybe $30? It’s been ages since I’ve done this. I’m not sure if any fees are incurred by online changes.