I have been using non-www redirect in .htaccess for a couple of years now for my Wordpress site. I want to switch to redirecting to www because, among other reasons, a naked URL does not work with Cloudflare CDN. Will it hurt my SEO score?
As you'll be using a redirect from non www to www all the previous incoming links and URL's will still be working and pointing to the correct content, so your SEO scores should not be affected.
The key here is that when ever changing URL's you have to have redirects (301) in place, so you wouldn't lose any SEO :)
I'll just in case mention that you do use the redirect, as all your site's pages should only either be www or non www, otherwise it could be classified as duplicate content.