Hi Archie,
Word 2003 is using Office XML formats, which are XML-based document formats. Office XML formats includes Excel and Word formats.
Word 2007 is using Open XML formats, which compressed several XML-based parts of an Open XML document within a zip package.
There are many differences between the two, you may take a look at Office Open XML introduction and Office XML introduction, which are talking about file formats, limitations and differences as well as examples.
Hope it will help
The two file formats are very different. Office 2003 document is stored in an integral file, but docx document is stroed in a ZIP package and organized based on "part" concept.
The name of some tags might be similar, but in a bigger picture, to distinguish the differences between those tags are daunting and not so helpful.
If you just want convert Office 2003 documents to Office 2007 documents, convert tools may be helpful, like http://www.download-up.com/free-computer-convert-word-2003-to-word-2007-software-downloads.html and http://www.labnol.org/internet/office/word-2003-convert-doc-to-docx-file-format-online-zoho-writer/2422/.
BTW, in my opinion, generate Open XML document from your data source should be the straightforward way. I am interested in your data source. Is it database or scratches from online input?
Thanks!