It's not possible to switch a worker to a user, you need to add him as a new user. To add an employee who manages and has access to your digital farm, mark “Can login” checkbox, enter its email and password and give it a role, depending on the scope of the permissions you want to give.
You can archive the worker so that all the data will be present in your account and just add him as user, but I suggest you to add some letter or sign to the name or last name so that you know in the reports which of them is a worker and which one a user.
To see how to archive the worker, click HERE.