can you buy-up a ticket after initial purchase/ticketing on NZ?
 

As background, we're limited to purchasing economy tickets for work and have to book them via a specific travel agent rather than directly through the airlines. On UA, their systems allow me to purchase the coach ticket via corporate, then call into reservations and buy up the fare. If it's done within 24 hours and you're buying up to a higher cabin (i.e. work paid for coach, I cover the extra $$$ to business), they'll waive all the change/hassle fees and reissue the ticket, charging only the difference in fare between whatever the original ticket was and the cost of the new one. Does NZ allow something like this? Given my experience with this sort of thing on various carriers, I'd believe anything from "of course they do this" to "are you insane, no way they do this!"

I'm coming back from AU next month and the SYD-AKL business fares are a very reasonable premium vs coach, so I wanted to get a small taste of the new NZ 789 premium cabin... ;-)

On TT routes you can typically upgrade your product without paying any rebooking fees. The same rules won't necessarily apply longhaul.

This does not appear to be a written rule so of course YMMV.

There is also the option of bidding for upgrades as well which is called 'OneUp'. If you put in a decent bid you can get a good chance of an upgrade.

Good luck with that. Every time I've deliberately attempted that they've changed the aircraft on me

Is this a Au NZ only ticket or part of a USA-NZ-Au return ticket?
As booked with a corporate TA it may be harder with Air NZ.

For all that SYD-AKL is about 3:00 hr flight time. In economy/coach paying for an exit row can improve the comfort.

When booked directly with NZ you can do this. You may even be able to do this online although sometimes I find the price is more than the fare difference so end up calling.

When booked through a travel agent the travel agent wanted to charge me fees when Air NZ didn't! In general I've been referred back to the travel agent for changes. Only after the 1st flight of an itinerary was taken would Air NZ then takeover and I could liaise with them directly. I've largely taken over my own bookings so haven't been in this situation for a while.

I would try your luck though.