Email address already used

Hi,

I just upgraded hikashop version from free to essential for the website so that there's no need for registration when creating an order. Since there is no need for registration now, everyone could purchase as a guest.

The website doesn't let the customers who have registered before use the same email to order as a guest. I've read through similar topics and learned that if you use the email that haven't been used for registration before, this will work. But the problem here is I was using the free version for quite a while, so there was quite a number of users who have registered during the time. So, is there a way they can use the same email to order as a guest without Email address already used problem? Like removing them from being a user? Would that help? I don't want all of my customers to sign up a new email in order to purchase a product from my website.

Thank you