Even if you changed it from private to non-private and had them posted only in a specific thread tag it probably wouldn't help, since I don't believe the system will reparse all the old apps.
Close, but not quite. Switching to "Non-Private" simply removes the private flag from the application threads. So in a sense it is retroactive. However, if you make your applications "Private" again, it will remember the original settings and those who had access to the thread at the time will only have access again.
In short, when in "Private" mode, there's no way to change who has access to existing apps. Removing private mode allows you to control which rank has access to all the applications (regardless of when it was created), and this is done by controlling the settings of the Applications Thread tag (or rather, the thread tag into which applications go).
Does that make a little more sense?
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It's all in the reflexes.