User Titles Don't Update With Usergroup

[CAAC]

I'm currently using this code to change the usergroup based on input to a custom field. This works, but for some reason the user title doesn't change with it. Instead the user title that shows up is the user title for the default registered user type. I'm using the register_activate_process hook.

Any ideas? Thanks

PHP Code:

// Get the value for field 15
        $user = $db->query_first("
            SELECT field15
            FROM " . TABLE_PREFIX . "userfield
            WHERE userid = " . $vbulletin->userinfo['userid'] . "
        ");


        if ($user['field15'] == 'yes')
        {
            $userdata->set('usergroupid', 18);
        }