Currently only moderators can do this, but I looked at the code and I think I can make a custom addon to the forum software to support users being able to move their own resources around as needed. This could then accommodate some cases of doing timed exclusives where items are offered to backers for a period before moving to free. By moving a resource, you retain the discussion history and ratings. The alternative is to make 2 resource posts, one in free and one in paid, but this splits the discussion and ratings so is not ideal.
