If you mean by using the PHP include or require commands, yes, those will work just fine, meaning that it will load the .html file no matter what you put in .htaccess. The reason they won’t be affected is because what that command I showed before does is create a rewrite rule for mod_rewrite that affects only browser requests. When you use include or require from your PHP script, that all happens inside PHP, so it’s not a browser request, and therefore is unaffected.
It will generate a 404 Not Found error. The reason is that mod_rewrite rules normally operate without knowing anything about the files that actually exist, living in a world of pure string manipulation based on what the browser requested. However, if you want it to fall back on the old htm file if there is no PHP file yet, check out the technique described here in the section titled “Backward Compatibility for YYYY to XXXX migration”
mod_rewrite is a pretty powerful tool and you can do a lot of really nifty things with it. You might want to take a look at the documentation to get a better idea of how it works and what you can accomplish.