I agree with the person asking the question, that there is a good reason to WANT to do this. I just gave my daughter a new PC with Office 2010 on it (she previously had Office 2003). When she goes into WORD and selects Open File, she can't see any of the files that she's used to seeing (because, the filter is set to look for docx files). Same thing when she goes to save a new document, she can't see any of the existing ones. I know, I told her to change the filter, but I can see that is a pain to do each time.
UNFORTUNATELY, Ken is right. If you rename a file that is really in .DOC format (such as from WORD 2003, or WORD 97) to .DOCX then WORD 2007 and WORD 2010 will give you a format error when you try and open them (it does not see that it's an older format, it just rejects the request)