Replace host in Uri

System.UriBuilder is what you are after...

string ReplaceHost(string original, string newHostName) {
    var builder = new UriBuilder(original);
    builder.Host = newHostName;
    return builder.Uri.ToString();
}

As @Ishmael says, you can use System.UriBuilder. Here's an example:

// the URI for which you want to change the host name
var oldUri = Request.Url;

// create a new UriBuilder, which copies all fragments of the source URI
var newUriBuilder = new UriBuilder(oldUri);

// set the new host (you can set other properties too)
newUriBuilder.Host = "newhost.com";

// get a Uri instance from the UriBuilder
var newUri = newUriBuilder.Uri;