Editing screenshots in iTunes Connect after iOS app was approved
Solution 1:
The current standing on this has changed to:
Beginning January 9 [2013], app screenshots will be locked in iTunes Connect once your app has been approved. New screenshots may be uploaded when you submit a binary for an update to an existing app or a new app.
Solution 2:
Apple support now got back with the (somehow not too satisfactory) answer:
If your app is currently for sale on the App Store, you will need to submit an update in order to change your app screenshots.
If you have any further questions regarding this, please let us know.