The subordinates of one Immediate Supervisor (Report To) can be changed to another through the 'Employee Reporting Re-assignment' page under Personnel Administration module.

Personnel Administration -> Employee Reporting Re-assignment 

Select the Report To employee whose subordinates required to change, select New Report To employee and the effective date. On selection of Report To and New Report To the subordinates of both employees will be listed in the below read only list. The same subordinates details list below again to select and move employees from old report to person to new report to person.

Do the required changes and click on Save button to take effect. 

If the effective date is not a future date then take effect on confirmation button click itself. If it is future date then this will get effective on that specific date based on the SQL job configured in the server.

Click below link to view create Employee Reporting Re-assignment Video: