This really would be the right thing to do. If it seems that most people not opt into the (old way) or opt out and stay out after a while, then maybe switch over.
The people of the internet are what make HN. Let them decide. Don't take the freedoms on hundreds of thousands of people away overnight.
There are only 5,500 people with 1000 karma or more. Most of them live in California. Now letting them agree with and approve the opinions and viewpoints of the other hundreds of thousands of members is going to shut down most opinions before they are even heard, some of which are more generally popular than theirs. The minority will silence the majority.