I returned an item that I purchased using a BearBuy Purchase Order (PO) but the PO is now closed. Do I need to submit a BearBuy Change Order Request to reopen the PO and process the credit memo? 

No, you do not need to submit a Change Order Request to reopen the PO to process the credit memo and the department does not need to submit the credit memo for processing. If the supplier submits invoices through Transcepta, they should send the credit memo through Transcepta as well.