Modify otherrefnum reference number on fulfilled sales order.

We have need to modify the reference number of a sales order which we populate with the customer purchase order number. Some of our customers use temporary po numbers to get items on order and then follow up with a number provided by procurement. We have instances where a special order may be received in our system prior to the customer providing the final PO number. It seems the record is locked after a partial fulfillment. Is there anyway to update the reference number after fulfillment? Would a scripted update also be denied