If you keep the pages in their HTML format, you can change the look without losing anything at all, of course.
If you decide to go to WP format you can use 303 redirects on the pages so that the old pages lead to the new pages. In some cases, depending on how you set up the permalink structure in Wordpress, you can make the permalinks the same as they already are. It will just take a little patience.
The biggest advantage of switching to WP is maintenance becomes a lot easier. Future changes will be easier. Adding to the site is easier. It just simplifies things and adds additional functionality.