Hello @CarolynK975 ,
A contact wouldn't really be able to change their email address, since that's what denotes a unique contact as it is. They can add their new, preferred email address as a new contact, via a landing page form or a coded form you've installed on your site. Landing page forms generate their own unique URL that can be added to your email, and coded forms would just use whatever URL your site provides.
If the contact goes through this process to add their preferred address, then they'd count as a new subscriber for automated subscription update emails that you can set yourself up to receive.
Nothing would inherently prompt the system to remove their old address. They'd need to notify you to remove it, or they'd need to explicitly unsubscribe that address, or you'd need to manually remove it of your own volition.
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