Yesterday, after reviewing the days we planned to be in New Zealand and looking at the 3x/week train schedule between Auckland and Wellington (and the ability to book a stopover on the train trip), Lisa and I concluded that if we put one more day onto the trip and rearranged our travel Auckland-Wellington, we have a chance of going over to the South Island for a few days after the convention an still go back up north and take advantage of the four-nights-for-three IHG offer in Auckland at the end of the trip. So I called AirNZ and they said that because it was within 24 hours, we could make the change for only US$100 total, which was acceptable.
In 2017, we had to make a one-day change like this because we'd misread the ferry schedule between Germany and Finland. In that case, we were less than 20 minutes from the 24 hours running out on the ability to make changes, and I was starting to wonder if IcelandAir would take the time we called (as opposed to when they picked up the phone after a long wait). In the end, they did accept that we were (barely) within 24 hours and made the change.
Unfortunately, the Coastal Pacific train south from Picton to Christchurch doesn't run in the winter, so if we want to go ride the TranzAlpine, we'll have to endure what appears to be a six-plus-hour bus ride.
There are other interesting potential trips that The Man in Seat 61 discusses, and the Taieri Gorge Railway looks fantastic, but I don't see any way to make it happen. A pair of six hour bus rides is bad enough, but another set between Christchurch and Dunedin? I don't think so.
Still to come: booking a hotel in Hamilton, sorting out how to get to Hamilton to the Hobbiton tour (preferably the evening one with the dinner) and all of the South Island travel, plus booking the Auckland-Wellington train (with a planned stopover at Hamilton).