Changing my homepage file extension - Google issues?

Hi

This is 2 questions really.

I am upgrading my very static website to something more dynamic.

Currently I have an index.html, plus quite a lot of static services pages which rank OK in Google.

I have installed a blog in a /blog directory and want to show recent posts on my homepage, which would mean changing from index.html, to index.php.

So question 1 is would this affect my Page Rank (currently 4) of the homepage/damage my Google listing/ranking

Second question, is whether adding a .htaccess file to re-write the URLs of my static site would also be bad, e.g changing from

domain.com/my-service-page-1.htm

to

domain.com/my-service-page-1

Do these old URLs (domain.com/my-service-page-1.htm) then become broken i.e. give a 404 error?

Thanks alot