A client's website is currently at www.domain.com, with domain.com redirected there automatically. I've suggested that they reverse the redirection to drop the "www", so I can set up a subdomain that can use shared session cookies (= login to domain.com stays logged in at sub.domain.com). However, they're concerned that the change will impact their search engine rankings. Any advice?

A client's website is currently at www.example.com, with example.com redirected there automatically. I've suggested that they reverse the redirection to drop the "www", so I can set up a subdomain that can use shared session cookies (= login to example.com stays logged in at sub.example.com). However, they're concerned that the change will impact their search engine rankings. Any advice?