The easiest way to solve this is to change the other account's email to something else (any other random email).

This message means that there's already another account in our system using the email address that you are trying to change it to. Most likely this other account has been set up by you.

Log in to the account that has the email address you're trying to switch to and change its email to something else from the settings page. That email address will then become unassociated with any account and you can use it.