I think you may have a unique case on your hands. I am a web developer for a big eCommerce site and we use emails to identify users (we switched from usernames to emails after a few months of testing actually). After three years of the site being up we have only had a handful of people get confused about their email changing.

If a user does change their email they can log in as their old email and simply update their account to their new one.

Email is a much better system than username because people constantly forget the unique username they created for that specific site. You don't really forget your email very often.