how to change widget type of title field in content type Drupal 7
i think, it is better to remove the original field 'title' and create new field for node reference type, use:

&lt;?php
    function your_module_form_alter(&amp;$form, &amp;$form_state, $form_id) {
        if($form_id == 'your_node_form'){
            $form['title']['#access'] = FALSE;
        }
    }

?&gt;