It maps multiple emails to the same identity, in that you use the same password to log in with all of them. The end site gets the email you log in to the site with (I can log in to Persona with email A but let all sites know email B every time. If I change that to email C, sites will think I'm a different user).

You can let users change email addresses, and nothing bad will happen. They'll just have to log in with the other address next time.