If I have 2 same name taxonomy term which is used with nodes. how to set only one instead of both?

Question

I have 2 taxonomy terms with the same name (knight(2) and knight(6)) in my Drupal 8 Project; both terms have been used with nodes.

I want to replace knight(6) with knight(2) in all the nodes. I created the following custom form.

The submission handler code is the following one.

  public function submitForm(array &$form, FormStateInterface $form_state) {
  
  $values =  $form_state->getValues();
  $old_term = $values['old_taxonomy_term'];
  $new_term = $values['new_taxonomy_term'];
  $query = \Drupal::entityQuery('node')
    ->condition('type','a')
    ->condition('field_taxonomy_', $old_term, '=');
  $results = $query->execute();

  foreach ($results as $nid) {
    $node = \Drupal\node\Entity\Node::load($nid);
    $abc = $node->get('field_taxonomy_')->referencedEntities(); 
    foreach ($abc as $key => $value) {
      $value = reset($value);
      if ($key['tid'] == $old_term) {
        $node->set('field_taxonomy_',$new_term);
        $node->save();
      }
    }
  }   
}

The code doesn't work.