I asked this question on the Apple Developer Forums as well: https://devforums.apple.com/message/965324#965324
You can't "delete" the current version. You would have to submit a new version and then go into the previous version list and disable the former version.
Okay, but would this enable iOS5 users to downgrade to the previous release that worked for them?
No. The user would have to delete the app from their device then download the latest compatible version to their devices.