The host is not typically sent in the HTTP response headers to the client browser. If you want the browser to retain the original uri, just change the host header and the uri on the browser will not change from it's original value.
If you have urls in the response payload that you need to change back then that's another story. You'll probably want to save the original url as a global variable and then do a search and replace on the response payload.
You might want to take a look at this recent blog I posted about a recent thread regarding modifying the uri and changing the response payload.
http://devcentral.f5.com/weblogs/Joe/archive/2005/07/27/1398.aspx
Click here
-Joe