Air New Zealand Air Points - Paying for Upgrades - with cash not Airpoints

We (boyfriend and I) are flying LHR-AKL in a month or so and are booked in Economy (not sure what the class of booking is) and a friend has just flown Air NZ and has raved about the new Premium Economy. I have contacted the travel agent we booked these through and also Air New Zealand themselves to see if we can change our tickets and they have both said that as the flight is ticketed we cannot change it.
I was just wondering if it was possible to upgrade to the next class by actually paying the difference on the day of departure?
I am an airpoints member, but just Jade (with very few points at the moment as I only joined two months ago!).

Thanks for your help.

Welcome to Flyer Talk Flyr :)

That doesn't sound right. Airlines are usually happy to take more money from pax, as long as there are enough seats left. Try ringing the airline directly to get a quote for upgrading your ticket. Expect to pay fare difference and change fees. Depending on your booking class and current premium economy availability the fare difference could be very large.

I agree with Kiwi Flyer. Almost any ticket on NZ can be changed "up" to a different booking class/travel class (if available) on the same itinerary for a fee and fare difference before the flights are taken.

UA offer their premium frequent flyers upgrades for cash at check-in or at the gate, usually for far less than what it would have cost to purchase the upper class seat in the fisrt instance. Sadly, no such opportunity on NZ :(