using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

          var pattern = new Regex(@"[^!]+!([^.]+)[^!]+!([^.]+).*");

          var input = "= Fields!Change_Date.Value & Fields!Change_User.Value";

          var output = pattern.Replace(input, "$1 && $2");

          Console.WriteLine(output);

    }

}

​