You might consider grandfathering existing posts to list in points groups for a brief duration just while everyone gets settled in. OR give us the ability to post only specific chapters to new groups. That way, we can pay for only new content to appear.
We can grandfather some content in. Maybe for the next week. If you want that, let me know which group you want that for.
You can also do the other alternative. The best way to publish select chapters to a different points group is to create another version of the manuscript and only publish those chapters to the points group you want. That way, you only have to pay for those chapters.