That probably means they are holding those premium center orchestra seats until they determine how high demand is, and will then sell them for higher prices. They were selling the rows in front of those, behind them, and on the sides last I looked...
I don't think better seats will be sold for the public sale. As for getting shut out, it's unlikely. Some shows hold a lot of premiums and have to reduce the price to regular a day or so in advance, if they aren't moving. They hold other seats back for people with accessibility issues and, if they aren't sold a few days out, they get released as regular priced seats.
But if this a cornerstone of your first trip to the USA, you should probably buy something in advance and be sure you're covered. Telecharge is usually good about giving you one exchange, so if better seats open up later, just give them a call and they will cancel your old order and ring up a new one (of course, if they are charging more for seats at that time, you will have to pay the overage).