Approval workflow in Purchasing

We want changes to our purchase orders that result in an increase in the value of the PO to require a re-approval.  The control needs to be in place only for changes increasing the value of the PO, not any other changes such as changed dates, changed distributions, updated part numbers etc.  The Oracle Purchasing Help document "Workflow Approval Overview" includes the following section: “Do Changes Require Reapproval? (Process) - This process determines whether the changed components require the document to go through the full approval process (find the next approver in the approval hierarchy and send notifications, and so on) or if it should be approved immediately. If the changed components require approval, the PO Approval Process begins.” How do you