I thought I had replied to this ? Something must have gone wrong
Anyhow, the simple answer is YES, as you have changed the extensions. unless you rewrite your HTML files to be parsed as php. I have done the same on my own site
Internet Marketing Consultant | Google Expert | Search Engine Optimisation Consultancy | Internet Marketing Consultancy | although all the pages there are being delivered as .htm extensions, they are in fact php pages and dynamic.
I suggest you do the same. Also you are going to have to make sure you 301 redirect links to any pages that will no longer exist.