OP, you are traveling in a few years, if I read correctly. A lot can change between now and then. I wouldn't count on the schemes working the same way.
If you were traveling in the next year, I would suggest trying to obtain global upgrade certificate from a United 1K. That would allow you to upgrade from a W fare or higher.
You can also use miles to upgrade from any fare, but they would require a copay.
Both these are kosher for the
UA metal flights.
To upgrade on a
NZ metal flight, you would need to purchase a full Y or a full B fare.