SkyTeam - Best strategy for upgrade - US to Auckland, NZ?

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I am DL GM and hope to upgrade on an M fare. Willing to fly from anywhere stateside.

Best way to try?

There isn't a way to do it that I am aware of. The only SkyTeam carrier serving AKL is KE (only 5x weekly) and you'll have to buy a Y or W fare to upgrade into the I bucket. Details on the upgrade program are in this thread (http://www.flyertalk.com/forum/skyteam/892072-skyteam-mileage-upgrade-program.html).

How about to Sydney?

DL is starting service to SYD from LAX in a few weeks. You're up for the typical DL upgrade process on those flights. There isn't any history to them so we cannot really say what the frequency of upgrades clearing are or what the Z (or is it I??) inventory looks like. But at least that should be a possibility.

Unfortunately I'm flying in May / June so DL to Sydney isn't an option.

Unfortunately I'm flying in May / June so DL to Sydney isn't an option.
Then you are back to the same place you were with the AKL destination. You'll either have to buy into the front cabin straight away or fly into the back I'm sorry to say.