What you could try is to possibly transfer it to someone else... worked for me a few years ago when I had 4 months left and qualified to Gold (and Elite shortly after). I called them up and asked if there is anything possible to refund or transfer the remaining time. They said the only thing they could possibly to is transfer to another person (no refund, no holding/freezing) which I was happy to do and made me my partner at the time also happy ;-)
But that was 4 years ago with 4 months left and it felt it was more one of these “having the right Air Nz person with all special computer codes” on the other end rather than it’s a normal thing to do.... not sure if that still works today, especially with the declining service and quality of the NZ FF membership.
Give it a go... but don’t expect it to be a garantueed thing.