Often times, clients want to revise an existing purchase order for quantity, price, etc. If a PO has been tagged as Open - any changes should require a revision - tracking it against the original PO number. Possibly provide the ability to prompt the user "do you want to create a revision ID for this change?" - if yes, create the revision ID (sequenced #s), if no, then just modify the original PO without creating a revision ID. Either way, PO revision history should be tracked.