[Admin]

In index.php replace this:

PHP Code:

$unregwelcomemessage='';
if ($bbuserinfo['userid']==0) {
  eval("\$unregwelcomemessage = \"".gettemplate('forumhome_unregmessage')."\";");
} 


with

PHP Code:

$welcomemessage='';
if ($bbuserinfo['userid']==0) {
  eval("\$welcomemessage = \"".gettemplate('forumhome_unregmessage')."\";");
} else 
  eval("\$welcomemessage = \"".gettemplate('forumhome_regmessage')."\";");
} 


Now delete $unregwelcomemessage from your forumhome template and place $welcomemessage instead.
Also create a template named forumhome_regmessage and use it for messages for logged in users.