This works with most user script plugins like GreaseMonkey, ViolentMonkey or TamperMonkey:

  // ==UserScript==
  // @name BackToOldReddit
  // @namespace Reddit
  // @match *://www.reddit.com/*
  // @grant none
  // @run-at document-start
  // ==/UserScript==
  const currentURL = window.document.location.toString();
  
  if(currentURL.includes('//www')) {
    const newURL = currentURL.replace('//www','//old');
    window.document.location.replace(newURL);
  }