How to modify comment reply form?
First of all the reply form and the comment form are the same forms (form_id: comment_node_NODE_TYPE_form), you shouldn't think they are different forms.

Second, you would need some custom coding in your own module, that would look like this:

/**
 * Implements hook_form_FORM_ID_alter().
 */
function yourmodule_form_comment_form_alter(&amp;$form, &amp;$form_state, $form_id) {


OR, even more specific to comments on article nodes (you can replace article with page, blog, YOUR_CUSTOM_CONTENT_TYPE) 

function yourmodule_form_comment_node_article_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if (isset($form_state['comment'])) {
    $comment = $form_state['comment'];
    // if this is a reply, hide the two fields
    if($comment-&gt;pid &gt; 0) {
      $form['subject']['#access'] = FALSE;
      $form['author']['#access'] = FALSE;
    }
  }
}


Good luck!