Does adding WWW to an existing site hurt SEO badly?

I have a client with a fairly large website, industrial with lots of customers and visitors.

But they want to add WWW back into their domain name, because they have an internal directory at their offices that uses the exact same domain as the website URL.

So just adding WWW to the domain name, and setting a permanent redirect.

I can do that - but wondering if anyone knows how badly it will affect the SEO rank? I know SEO has changed alot in recent years, even months.

Thanks