public function substituteGroup($old, $new)
    {
        $groups = $this->getGroups();

        if(($key = array_search($old, $groups)) !== false) {
            unset($groups[$key]);
        }

        if (!in_array($new, $groups)) {
            $groups[] = $new;
        }

        $this->fields['GROUP_ID'] = $groups;
    }