They are not sold out. There are still 300-400 available for 12/31, and over 1000 for 1/1. You can pull up the shows on Ticket. and drill down to see what seats are still available, and there are plenty up on Stub Hub too!
I have tickets to both and I am concerned about the 1/1 show happening. I believe a lot of people are staying away from this show with it being so expensive, a major holiday, and it is still unknown if Nick will be playing or not.