Here's an idea.
Copy out all the posts(either as documents or html), reset the course, then set up the forum the way the instructor likes it, putting back in all the posts. Now before any students are enrolled, go inside the forum and back it up from the forum administration area of the administration block including all user data. Save this backup file hidden in the course for later use. (I would recommend putting it in a hidden topic so it will be orphaned and not as easily deleted by the instructor.) Then the next time you reset the course, delete the used forum which no longer has the instructors posts and restore your saved backup file of this forum. This will put back in all the instructor's wonderful posts fairly quickly. Depending on how many there are in the course it may take a little time, but less than redoing all the posts if they have to be put back in each time. If there are quite a few of these forums, you might want to do a whole course backup, choosing only the forums, so that they can all be restored all at once instead of one at a time.