You cant fix it. Your users all exist in phpbb2 or the new phpbb3. There is no specific way to fix it other than coming up with a proper way to integrate.
You can reinstall using your old files as a standalone using the install file but that is going to mess with your site and photo ownership and users would need to reregister and then when you hook things back up again lots of work to get everything back to a new integration.
You can role your forum back to phpbb2 before you upgraded by reverting your forum files and database.