You need to do a blank initialisation for the new pay group if your old pay group is in the latest period. You may follow the steps below:
Go to Process Pay and click Initialise.
Select the new Pay Group, e.g., 1 Payment Group.
Click Initialise, click Next, and you do not need to select the employee over.
Repeat the Initialise step until you achieve the old pay group's current period.
Then, you can assign all the staff to the new pay group as follows:
Go to Payroll, and click Setup.
Select Pay Group Setup and click Assign Employee.
Select the new pay group, and move the employee over.
Click Confirm.