Webassessor login & email Address update issue.

You can change most details on the Edit Profile page, your e-mail address is something you can't change. You'll need to contact them directly, you can use ktnsupport@kryteriononline.com to ask them to change those details.

How do I get all of these to be updated to my personal email address (at both Salesforce & Kryterion) without impacting my certification data/account?

For clarity on this, your Salesforce and Kryterion aren't one and the same, they're totally seperate. Changing your Kryterion address will not affect your certifications. What it will do, however, on Salesforce's Verification site is change lookups to your new address. So if someone is used to typing your.name@company.com and you change it to your17name@gmail.com, they'll find nothing for the former but your certs for the latter.

Tl;dr:

To change your e-mail on Kryterion, you have to contact them.