No need Greg. It's a software architecture problem in UBB.
Here's the cause. When you make a forum "private" (let's assume this is Forum #1), the data files for the private forum are tuck under a randomly named directory directly underneath ubb/Forum1 which contains the data for each thread (call this directory where private threads are stored as ubb/Forum1/4jdsk04). When you change this Forum #1 from private to public, UBB now reads the threads files from ubb/Forum1 - it doesn't move files in 4jdsk04 into ubb/Forum1. As a result, it looks as if the private files have just disappeared (the data files will still be in 4jdsk04 though); they are not moved when you change from Private to Public.
Therefore, the softare architectural answer (the easy answer):
1. If you switch the forum from private to public, authorized users will only be able to view/add/edit/delete/... the public directory, not the private directory.
2. If you switch the forum from public to private, authorized users will only be able to view/add/edit/delete/... the thread data stored in the private directory, not the public directory.
Please LMK if you have any other questions.
qasic