Change orders can only be accommodated for Contract and Vendor Blankets in Gateway using the Contract Amendment or VB Revision Forms. If an invoice is in-process or paid the CCOA cannot be updated and a new PO will need to be created if the CCOA needs to be changed.
If you have a budgetary change for standard purchases ("SBs"); this would be handled as a TOE or cancel the order and reissue a new order if the vendor hasn't fulfilled the order yet. These will be treated as TOEs.
If the supplier responds with an unavailability of an item, or a substitute item; these changes can either be made on the Receiving side of the order, or treated with a cancellation of the order and reissuing with the revised item(s).