By using a rewrite you can continue to use the same URLs for your pages (URLs are defined by what is in the links *on* your pages) and 'associate' that URL with a different physical filename on the server.
I use PHP a lot, and there's also a way to tell the server that even though the filename ends .html to still go and look inside it for PHP code. With that method, you don't need the rewrite at all.
So, there's at least two ways to achieve this; neither of which see you having to change the URLs that users 'see' and 'use'. That is always a better method than changing URLs, with all the attendant ranking and traffic losses.
See also "Cool URIs don't change" by Sir Tim Berners-Lee.