....Do not log out before you re-enable the board, or you will not be able to log back in.
That's odd. I've used Invision before and it certainly was not like that. I would have assumed the same for PHPBB. What you could do instead is place an index.htm file in the root stating the forum is offline. Or copy index.php and save with a new name and edit index.php to tell people the forum is offline, then replace the original index file when you're ready.