Many organizations are now using Office 2007 or Office 2010 most documents are still in legacy format. Although there is no harm with using doc files created in previous versions. But there are limitations with formatting options and manipulation of the overall document.
Microsoft provides with a tool that can help converting many doc file and a fileshare or a multiple directories to the new docx format.
here you can download tools for the relevant version of Office
with OMPM (Office Migration Planning Manager) one can batch convert old doc file to docx format.
Instructions to Migrate Word, Excel, and PowerPoint files to the 2007 Office system can be found here
Instructions to Convert binary Office files by using the Office File Converter (OFC) and Version Extraction Tool (VET) can be found here