Unassign the supplier user account from one party and assign to another

The situation is following: two suppliers belong to the same group but are different companies, and we have separate agreements for them. The contact person assigned to the supplier A has to be transferred to the supplier B - with the same email address. However, the contact has a user account.

We deactivated the user account and the contact under supplier A and are trying to create the same contact under supplier B. However, the system doesn't allow to create a user account for the new contact - it indicates that this email address is already in use:

An account for this supplier contact can't be created because a user account with this email already exists. (POZ-2130076)