How do I change the default value of a built in template variable?

/*
 * Inherit contentType from Parent
 * Written By Paul Suckling 26 Sep 2007
 * Based on "Inherit Parent Template by "Raymond Irving - 12 Oct 2006
 *
 * Simply results in new documents inherriting the contentType 
 * of their parent folder upon creating a new document
 *
 * Configuration:
 * check the OnDocFormPrerender event
 *
 * Version 1.0
 *
 */

global $content;
$e = &$modx->Event;

switch($e->name) {
  case 'OnDocFormPrerender':
    if(($_REQUEST['pid'] > 0) && ($id == 0)) {
      if($parent = $modx->getPageInfo($_REQUEST['pid'],0,'contentType')) {
        $content['contentType'] = $parent['contentType'];
      }
    }
    break;

  default:
    return;
    break;
}

global $content;
$e = &$modx->Event;

switch($e->name) {
  case 'OnDocFormSave':
    // Get the current alias
    $alias = $content['alias'];

    // Discard the extension if one exists...
    $dotPosition = strpos($alias,'.');
    if (!($dotPosition === false)) $alias = substr($alias,0,$dotPosition);

    // select the appropriate extension
    switch($content['contentType'])
    {
    case 'application/xhtml+xml':
        $extension = '.html';
        break;
    case 'text/html':
        $extension = '.html';
        break;
    case 'text/xml':
        $extension = '.xml';
        break;
   // Other contentTypes here...
    default:
        $extension = '.html';
        break;
    }

    // Register the modified extension
    $content['alias'] = $alias . $extension;

    break;
  default:
    return;
    break;
}