For a long time, because I once moved from Movable Type to WP, I was still manually uploading and inserting images into posts because they were all added to a /archives directory on the server. Last year I started using the WP media uploader, and I was afraid I had to assign feature thumbs for all the older posts.
Not! This plugin did it all for me. Took a while with so many posts, but no hickups and now all my posts have thumbnails.