Can I change the primary email address?
Yes,
you can change the primary email address. This feature is helpful in a scenario where administrator rights has to be changed from person to person, so that the account remains active throughout.

You can change you primary email address in Profile >> Email, Services and Devices >> Add email address

The new added email address will receive a verification email in its inbox which needs to be verified, so the "Make Primary" option is visible in Email, Services and Devices section.

New email address is changed to the primary email address

Is it possible to change the primary email address to an email address which is verified and registered with us?

Yes. But in such a scenario the primary email address holder will have to claim the email address which they would now like to make as primary.
Following would be the process:

  1. Add email address

2. Claim email address

3. Verification code sent to the email.

4. Claim message reflects as the verification code is submitted.

5. New Email address added in the section Email Services and Devices.

6. Use the link "Make Primary" to change the email address to the new one.

7. Notification Message. 

Can a non-whitelisted email address be added and made primary by a member?
No, You would be able to add a non whitelisted email address, however when you try changing the primary email address to the non whitelisted one it will give you the below warning message that your login will be blocked from the communities.

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