This proved a little tricky to solve in Orchard. There are some modules out there to do redirects, but they were built for older versions of Orchard and no longer work. They rely on intercepting the 404 using the <customErrors> tag in the web.config, and changing it to a 301. Unfortunately, this doesn't work in Orchard 1.7.2. If you specify a page in <customErrors>, you'll see that your page is never called.