How to add Customer PO Number in Recurring Bill plan contract in Receivables after Invoice created

Summary: Bill plans (recurring billing in Receivables) do not allow POs to be changed once invoice is generated in AR against plan. Currently a few customers that are insisting on providing a new PO quarterly and we are having to manually edit the invoice that are generated to address the requirement. With in the RB plan have PO Revision field, however it is greyed out once AR invoice is generated against Bill Plan.

Content (required): Some fields in RB plan allows to change however like PO/ PO Revision fields did not allow to change and these fields were greyed out after the AR invoice is generated against the Bill plan. Now a days Customer wants to change the PO number every quarter, but we cannot change it in the Bill plan contract that have been using past 5 years, either we have to manually edit every invoice created after po changes or Closing the old bill plan and creating a New Bill plan for Every quarter which is difficult for the customer.