Add another type to your httpd.conf or whatever server you use so that .html or .htm files are parsed just like .php or .php3
Hint! add .html to the Types and save .htm for good old html or something.
All you need to do then, is to save the website as it is.
You can then also have all your hyperlinks as .html, and if you at any point want to keep the .php, it's not a problem.
Good luck!