NZMC has already been upgraded, about 4 years ago, it was widened in order for Pionair to fly the Convair into and out of. Still has the terminal facilities which Mt Cook Airlines used to use, so not much needs to be changed.
The long term vision for NZUK is for a new longer and wider runway, but that plan is for about 20 to 30 years time! But in order to use NZMC you need a suitable alternate close by otherwise its a very long bus ride when the winds are too strong, or cloud/vis is too bad to get in (very frequent events) - hence why Mt Cook used NZUK.