Purchase order change approval tolerance problems

We have change order re-approval tolerances set on need by and promised dates.  When dates are changed, within the number of days set on the tolerance form, on the shipment records the purchase order status changes to Requires Reapproval as expected.  But when the Approve button is pressed the Approve Document form opens to submit the change for approval.  It's my understanding the order should change to Approved status and not invoke the Approval workflow.