Impact to existing employee if change Employee Identification to manual

We'd like to make the Employee Identification field to manual entry.  I found the steps how to do this, but am wondering how this will affect employees who already had an employee identification number defined.  When I made the change to manual, I queried an existing employee.  The field is now editable, but I need to know what functionality will be affected if I change this number.  Workflow, etc.
Thanks,
Patrice