Hello
A question similar to this one has been asked already please have a look at this post:
How to convert a .doc, .docx file in .html[
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It will work between doc and docx.
Also, at msdn someone has asked more directly the idea between word file
http://social.msdn.microsoft.com/Forums/en-US/f51bdd0c-b022-45c6-8af5-6ed79ad576fd/convert-doc-to-docx-?forum=vsto[
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I'm quoting here the example just in case this last link ever gets deleted
object oOpenName = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) + \\TestSaveDoc.doc;
object oSaveName = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) + "\\TestSaveDoc.html";
object oHTMLFormat = Word.WdSaveFormat.wdFormatHTML;
Word.Document doc = wdApp.Documents.Open(ref oOpenName, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);
doc.SaveAs(ref oSaveName, ref oHTMLFormat, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing, ref missing);
I hope it helps
Regards