Right now, iTunes Connect is pretty much broken. Sometimes your screenshots won't appear on the App Store, and you have to keep re-uploading them until iTunes Connect decides it wants to properly sync with the Apple CDN servers - even when that happens, often the screenshots order will not be the same as what you set.
Also, this is going to increase review times. Now devs will have to submit 'updates' just to change their screenshots.
What Apple should have done is offered an alternative screenshot review process, that does not require a new app binary to be submitted. Essentially, this current implementation just wastes everyones time.