Just now I have developed a small extension for PunBB that will let admins and moderators to change the ownership of any topic. As usual, I made it for Projanmo Forum but it is always pleasing to share with the community :).
It is very simple plugin. Just upload and activate the extension. Then if you view any topic with, at least, moderation privilege, you will see a link ‘Change Topic Owner’. When you click on it, you will be asked to enter new User’s ID (check the image below). Once you enter and hit Ok, you will be redirected to a new page where either the ownership will be changed or corresponding error message will be shown.
Note: This will not affect users’ post count.