Normally changing the path to / from empty will just mean that users will need to login again. Nothing you can do on the server side on this to clear out the cookies. To test, if you can make a backup copy of your config.inc.php file then you can set the cookiepath variable to / and see what happens. Try going to your forums and logging in. If it ends up causing problems then you can just copy your backup to the original to switch back quickly.
