Well, if your main concern is maintaining the original urls for the 160 or so current static pages you could use mod rewrite (assuming that your server is running apache) so that a server call for site.com/category/ID/article.html yields a url that reads site.com/article.html.
It would be "backwards compatible" such that if I put site.com/article.html in my browser's address bar (or if I do a search query that points me to your site.com/article.html url) your CMS would translate that url into site.com/category/ID/article.html and thus find the correct content data.
If you do a Google (or whatever other search engine you use) search for "mod rewrite" you'll find enough info to help you out.
The other possibility is seeing if your CMS handles this natively through some type of SEF url creation capability. In most of those cases you can manually create additional SEF urls. This would allow you to create the shortened url you're looking for.