The password is incorrect or an account associated with the new email address already exists.
Grammarly shows this error because an account associated with the email address you would like to use already exists. You will need to delete this Grammarly account to be able to switch your current account to the new email address.
- Log in to Grammarly with the email address that you wish to use as your new login
- Visit the Profile page and click on "Account" at grammarly.com/profile/account
- Click the "Delete account" link
Grammarly will ask you to provide your password to confirm the action. Once you do that, your account will be deleted.
After this free account has been deleted, you can proceed with changing the email address associated with your Premium account.