Need help with processing retroactive costing changes.

Content

Hello fellow experts

Can someone please guide me through step by step process how to retroactively change cost allocations for one employee.

Example

In the pay period there is and earnings that are allocated

40% - cost center 123

60% - cost center 234

I need to achieve:

30% - cost center 123

30% - cost center 234

40% - cost center 345

I know how to correct these allocations from the pay period where changes apply 

My question - what else (steps, processes etc) should be done after that changes are performed. 

Accounting periods are already closed as the changes I need to do are back in March-April time.