One reason for changing the GUID

There’s a filter for that.

I use:

function ts_feed_spruce_get_the_guid($content) {
  // double ? is ok as guid is not a url
  // '&' gets esc_url'ed and doesn't work anyway
  $content .= '?d=' . get_the_modified_time('YmdHisT');
  return $content;
}
add_filter('get_the_guid', 'ts_feed_spruce_get_the_guid', 7);

Drop it in your theme functions.php or a plugin.