confirming a new or changed email address?

This should happen the initial time we receive an email address for a user, and also each time a user changes their email address.

Any time a user enters a new email address or changes their email address, the site sends an email to the new email address asking the user to click on a link to confirm the address.

We have a way to track the most recent date and time a user confirmed their current email address.

We retain the old email address until the new email address is confirmed.