Esta página do blog do MSDN deve ajudá-lo.
Microsoft has a bulk conversion utility that can convert multiple DOC files to DOCX files. This blog post presents very basic information on its use. This utility has been around for a while, but a fair number of people don’t know of its existence. Because it’s a powerful tool designed to convert all files on a server (or even a bunch of servers), its use is a little cryptic. This is fine for IT professionals, but if all you want to do is bulk convert a single directory of DOC files, this post shows what you need to do.