If the same email address has been used as the root email address for a new AWS and for interacting with Amazon.com, those two accounts are inextricably linked forever.

Example

If you have an Amazon.com shopping account with email address xyz123@cornell.edu, and create an AWS account with that same email address (xyz123@cornell.edu), the Amazon.com account and the AWS account become linked (forever).

In this situation:

If you were to change the root email address for the AWS account, it would also change the email address for the Amazon.com account.
If you were to change the email address for the Amazon.com account, it would also change the root email address for the AWS account.
AWS/Amazon cannot manually unlink or decouple these accounts.
If the Amazon.com account is closed, the AWS account will get closed.