Heads up: this should be used if you have recently switched your programming of choice. Example: all of your .html file extensions have been globally renamed to .php
Simply copy and paste this into your .htaccess file.
redirectMatch 301 ^/(.*)\.html$ http://www.domaininquestion.com/$1.php
Make sure to upload this file to the folder you would like this change to apply to or just upload to root to make all files do it.
Warning of Eternal Damnation: make sure all of the file names are the same, otherwise, you'll get 404s. And this, will certainly lead you to a life of distinct disgust between you and your visitor(s).