In an email sent by Apple to iOS developers today, it appears unless you specifically set the flag on each app you have, users who downloaded your app in the past can now get the older versions that will run on older OS and hardware levels (considering you've updated your app to build to iOS7 and aren't supporting the older versions universally in your binary).
If you do not want previous versions of your app to be available, for example due to a usability or legal issue, you can manage their availability in the Rights and Pricing section of the Manage Your Apps module on iTunes Connect.'
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