Modifying Pre-Approved PO

Hello,

My client is using encumbrance accounting and has a purchase order which is Pre-Approved because the funding charge account does not have sufficient funds. They want to modify the charge account to an account with sufficient funds, but since the document is in Pre-Approved status, it cannot be opened. 

How can we get the status changed so that the system will allow us to open the document?

Thanks,

anthony