One other thing I forgot to say - if you do decide to switch FFPs (eg from UA to AC, because you don't think you'd still be able to manage 50K paid miles to keep your *G status), when flying on AirNZ there are actually two fields for entering FFPs: one for the program which you want miles credited to (say, AC) and one for the one you have status with, if it is different (in your case UA). This could be useful for ensuring you still get *G privileges while accruing status on a new FFP.

The bad news is that these numbers can only be entered by NZ reservations staff over the phone, and most of them don't know that the two different fields exist. Worth it for long haul though.