Impact to existing employee if chnaging Employee Identification to Manual

We'd like to make the Employee Identification field to manual entry.  I'm 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 already-defined number.  Workflow, etc.
Regards,
Patrice