Well yea if you're searching for LGW - AKL that's going to be difficult because it includes 3 separate legs, and as most people will tell you it's hard enough getting availability on one leg. My experience with EVERY single flight I've taken so far has been that upgrades open up from a day to a week before departure. As Silver you can get on the waitlist or request an upgrade at check in. Otherwise, just look for individual legs. If you've found one now, take it, because cancelling the upgrade costs you nothing and you get your miles back.