WOW, I was just about to ask the same question but did a search and am glad to find some information as recent as yesterday.

I just want to make sure I understand: I have a ton of Mileage Plus miles almost all from UA (with some AC). I can't use them to upgrade a purchased Air New Zealand coach ticket to business? I would love to take the new SFO-AKL nonstop...

If that's the case, I'm surprised, since I believe DL folks upgrade purchased coach AF tickets all the time even when not codeshare.

Thanks in advance for the clarification...the strategy of connecting through SYD on UA might be the only alternative...