Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
if (wgPageName == "Main_Page") {
  window.location = "http://en.wikipedia.org/wiki/Wikipedia:Main_Page_alternative_%28Classic_2006%29";
}