NZ bumped me to gold a day after a chat with customer service. And, realising that I can check expiring credits on my statement has been enormously helpful. Many thanks all on the forum!
This experience did bring up a related question - I took a trip in March 2015, but for various reasons it did not post until June, 2015. Will my credits expire on the anniversary of travel or anniversary of posting? If, by lucky chance, the latter applies, I'll have a decent chance making Elite this year.