In the iTunes Connect App Management interface -- how do I edit the screenshots for my localized (approved and live) iPhone app?
Unfortunately, the web upload form had a bug which actually required the screenshots to be provided in reverse order (I provided them in the correct order, which meant that Apple reversed them and now they ended up wrong). Also mentioned here at StackOverflow. I only managed to edit the 4 screenshots in the US version, but not my localized version, and that was in the old interface.
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.