I have defined Release Strategy for PO :
Buyer : 00, Mgr : 01, Sr Mgr : 02 & GM : 03.
Scenario : For a PO, Release affected is 00 & 01. Final released to Vendor.
Now for any amendment, client requires, 01 should not be authorised to change PO. Only 02 will be authorized to amend or reset PO. Then only subsequent changes can be done again by 00 & 01.