Need Advise how to Setup Purchase Order Approval ONLY when the PO is Modified

Hello Gurus,

I would like to seek advise how to setup Purchase Order Approval ONLY when the PO is modified. Currently, when a PO is submitted there's no approver required; the Buyer, when submitting the PO, it is automatically gets approved since the PO has a link to Procurement transaction which was already approved by the respective approvers.

The reason for this requirement is to control the PO changes being done by the buyers after the PO has been approved, for example, Supplier Name change. This will be used for Audit control, identifying which field has been changed by the buyer and will have the PO submitted for Approval.