React if user's text changes in existing node are major
I would use Diff and implement hook_entity_diff() (see diff.api.php), which allows modules to provide a comparison about entity properties.

Here you go:

function MYMODULE_entity_diff($old_entity, $new_entity, $context) {
  if ($context['entity_type'] == 'node') {
    $old_body = $old_entity-&gt;body[LANGUAGE_NONE][0]['value'];
    $new_body = $new_entity-&gt;body[LANGUAGE_NONE][0]['value'];
    $matching = similar_text($old_body, $new_body, $percent);
    $results['line'] = array(
      '#name' =&gt; t('Body'),
      '#old' =&gt; 'Number of matching chars: ' . $matching,
      '#new' =&gt; 'Percentage difference: ' . $percent,
    );
    return $results;
  }
}