Got the same problem – good to see you are working on it 🙂
I ended up doing it by editing the wp_post table directly
I should add: so far, all my posts have been a single image per post. Now I have switched to a format, where every post is empty, with a single featured image. In order remove duplicate content, I now need to remove the original content (the single image per post) from every post, but I don’t want to do it manually since I have 80-100 posts.
