User email changes verification strategy?

Hello, I'm adding some verification into my user database manager. After a while I come up with some ideas about asking user to verify their email address when they change their email address. But, the strategy is a lil confusing for me. When they change the email address. Where should the verification code be emailed to? Of course it's not the new addy, but what will happen if their old addy became unusable (so they change), can you guys come up with some ideas? I'm confusing... TIA