If it's hosted at a 3rd party site, then unless it's contained in the default pool, simply replacing the headers won't do much aside from "tricking" the existing backend webserver that the request came to it with a different URI. If you need to send the client to another resource that is not in the BIG-IP's load balancing configuration, you are going to have to rely on a HTTP::redirect which sends a 302 response back to the browser with a Location header specifying that it should re-request the page from the new URL.
-Joe