Hmm...I wouldn't bother making that particular change if I were you, but assuming you ahve good reason to do it, I would use command line.
The command "ren" will rename files. You can use wildcares. Make sure you know the path of the drive/folder you want to affect and eitehr navigate there with the "cd" command or type it in with the ren command. For example
Ren h:\documents\*.docx,*.doc
That should accomplish what you want.
But if you're trying to get documents to open on an old version of Word by changing the extension, this won't really work.