You used to have the majority files using an extension like .php and have changed everything to .htm so using a 301 redirect you can update all links to use the new extension.

All .php files redirect to .htm files, such as to

Follow below example of All files with certain extension.

RewriteEngine On
RewriteCond %{REQUEST_URI} .php$
RewriteRule ^(.*).php$ /$1.htm [R=301,L]