If you have lots of posts published without setting a featured image, how to manage it quickly if you need the featured image now? Just install the plugin Quick Featured image and you may find it’s really useful for you to achieve your aim for this plugin can add/replace/remove the freature image in bulk by filter of category/tag/post and so on.
The only one point need to notice is that this plugin doesn’t supply a method to deactive/remove it, but you can doing these by the FTP way and you’d better backup firstly and clean the WP after doing your actions.