Change a comment's body programatically
hook_comment_insert() is fired after the comment body has already been saved to the database, so it's probably not the hook you're looking for.

Instead you could hook up your own validation function to the comment form and alter the body there. That way you're not having to make any extra db calls.

In it's simplest terms you would implement hook_form_FORM_ID_alter() and add the validation function like so:

function MYMODULE_form_comment_form_alter(&amp;$form, &amp;$form_state, &amp;$form_id) {
  $form['#validate'][] = 'MYMODULE_comment_form_validate';
}

function MYMODULE_comment_form_validate($form, &amp;$form_state) {
  $current_body = $form_state['values']['comment_body'][LANGUAGE_NONE][0]['value'];

  $new_body = $current_body . ' some extra text';

  $form_state['values']['comment_body'][LANGUAGE_NONE][0]['value'] = $new_body;
}