After my site is switched from phpbb 3.04 to vbulletin, will the forums that require you to be a member of a certain group be open for everyone?
I really wouldn't like my moderator or admin forum be open for public at any time.
If you have your board closed while you are doing this upgrade/change, then no one will see a thing except the "This board is Closed" message. You can always open up a new browser (not a new browser window, but a new browser) and see what others will see on your site.