In the SEO Snippets Youtube series of Google Webmasters, John Mueller replied to James from UK on his question “Will Removing “.html” from my URLS help my site?”
”URLs are important for search engines like Google. Do they care which endings your URLs use though? The answer is no.
Google uses URLs to identify pieces of content. Whether your URLs end with .html, .php, .asp, or just have words in them, doesn’t really matter to Google. All of these URLs can show up in search in the same way.
That said, if you need to change your URLs, for example if you move to a new content management system that doesn’t allow you to use .html URLs at all, keep in mind that this change would be a restructuring of your website. You would need to redirect the old URLs to the new ones.
This kind of change can take quite a bit of time to be reprocessed. So picking a time when you’re not dependent on search is a good idea. And because it can take time we don’t recommend doing this kind of change on a wim. When making URL changes pick URLs that you’re sure can last a longer period of time.”
Here is the Video: