In my opinion, fine tuning the app metadata, including the screenshots should be possible to do at anytime, not just when you update the app. This annoying limitation no doubt saves work for Apple reviewers, but probably means a lot of store listings remain out of date since developers may not always have enough time to go through the whole process of updating the screenshots at the exact same time they roll out an app update.

This is useful, but only seems to be for reviews the developer has already responded to, not for new reviews.