Checkout as guest while having an account.

Hi,

A registered account has access to the order history, the address manager and other features on the frontend, which guest users don't have access to.
And, like Joomla, you can only have one user entry for one email address.
And you don't want to convert a user account into a guest entry with just the user providing the email address of the user.
That's why it is blocked.

If you were using registration and switched to guest checkout only, then you should delete the Joomla user accounts of your customers. That way, they'll be able to guest checkout, like the other customers.