C# code to convert Doc to Docx

This tasks becomes very simple if you can use a C# Word library.
All you need to do is this:

DocumentModel.Load("Input.doc").Save("Output.docx");

You can also do this to stream (aka download) an "Output.docx" file to your ASP.NET application's client:

DocumentModel.Load("Input.doc").Save(this.Response, "Output.docx");