What you need is a little bit more firepower. I found the same problem with .doc and .xls files (I wanted them to open in Pages and Numbers instead of Office’s Word and Excel), so I started using a Preference Pane called “Default Apps.”
What happens is each file has a creator code, and OS X won’t override the creator code for Word files through the document’s Get Info window, as you found. But Default Apps lets one override the creator code and force .doc files to open in other programs, like Pages.
Once you install the Preference Pane, open it up and go to the “Extensions” tab. Scroll down (through a long list) of file extensions to “doc,” and then highlight it. Select Pages from the Default Application menu, and also check the box “Override File Creator.”
Now, all your .doc files will open in Pages. And because Pages can export a .doc, you won’t have a problem editing a .doc file and then sending it back to someone as a .doc for editing in their copy of Word.
It’s a great preference pane with many capabilities. You can also go to the Apps tab and see what extensions are set for each application. And set default applications for email and web etc. Download RCDefaultApp from Rubicode.