The Payroll ID is an identifier that you can give to an employee. This identifier is sent to HMRC. An employees payroll ID will follow them between jobs, payroll software and even via their pension provider to identify them. For an employee being migrated over, you can enter the employee identifier from the old system.
You will have the option when adding a new employee to enter their payroll id as shown below:
Once the employee is in the system the Payroll id can be found by going to the individual 'Employee Details' > 'Tax and NI Details' page.
To add the Payroll id on the above page you will need to enter in the payroll id (before you have run a pay run and submitted to HMRC) and press 'save'.
If the Payroll id box is blank the system will send the employee id to HMRC which is automatically generated, you can see this ID in the URL and it can also be found in the employee's details page.
Leaving the payroll id blank will create a new employment record for them with HMRC which is going to be a problem. If you do this you would need to contact HMRC who will then provide guidance on how to correct it.
If the employee has previously been reported (ie you are changing systems and you are transferring their details to this system) you need to tick the box to say that this employee has previously been reported as well as use the employees original payroll id.
If you want to change the payroll id then you can do this as long as you have sent one submission to HMRC using the original payroll ID. As long as they have been reported at least once within the system you can then change the payroll ID. The system will detect this has changed and will add the appropriate flags in the FPS to notify HMRC.