As of my knowledge, there are no standard solution.
You may can check the flexible own development. By using an badi/exit you can do that.
The logic will be, whenever you will update the EBAN table, system will check the existing PO number associated with this PR number, then system will update the PO number (EKPO table) automatically as per your requirement.
Find a badi or Exit, then consult with your ABAP team.