Assign default terms while editing existing nodes

The taxonomy_default module does not provide default taxonomy terms for nodes already saved. This approach may be correct, in order to avoid that editing a node an unwanted term could be assigned to the node itself.
In my site I added a vocabulary to a node type, and made it "required" by configuration. I need to make sure that any time an user edits a node, the new default taxonomy term is selected if no term in that vocabulary has been previously assigned. I found out that such behaviour may be achieved with only minor changes in the form_alter() hook of the module.

Here is what I am proposing:

function taxonomy_defaults_form_alter($form_id, &$form) {
  // Only alter node forms
  if (isset($form['type']) && $form['type']['#value'] .'_node_form' == $form_id) {
    $node = $form['#node'];
    // Add the default 'pre-selected' terms to $node->taxonomy
    foreach (taxonomy_get_vocabularies($node->type) as $vid => $vocab) {
      if (variable_get("taxdef_{$node->type}_{$vid}_active", FALSE)) {
        // Do not preselect terms on nodes that already have been edited
        // except in case the term is required by configuration
        if (!isset($node->nid) || ($vocab->required && !taxonomy_node_get_terms_by_vocabulary($node->nid, $vid))) {
          $default_tids = variable_get("taxdef_{$node->type}_{$vid}", array());
          foreach ($default_tids as $default_tid) {
            $term = taxonomy_get_term($default_tid);
            $form['#node']->taxonomy[$default_tid] = $term;
          }
        }
      }
    }
  }
}