If you look at the detail tabs in the Xcode build results window after building Ad Hoc, you will notice that the last few steps include starting the ProcessProductPackaging and CodeSign utilities.
If you copy these command lines (and intermediate and subsequent steps, if any), you can manually run these commands (or from a shell script) by replacing the new mobileprovision file in the appropriate place to create an updated ipa file without recompiling.
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