How to add PO Number to the Identifying PO field after Invoice creation

Summary:

We have a scenario where invoices are entered as a Non-PO Invoices but eventually, they have to be tied to a PO. This could be for various reasons (e.g. the Supplier did not send the Purchase Order on time).

Once the invoice is validated, the "Identifying PO" field appears to be un-editable.

Is there a way to edit this field or to even set the status of the invoice as Incomplete? Is there any configuration that we are missing?

Having to cancel/delete and rekey the invoice feels inefficient.

We have already suggested the canceling the line option to business, but that doesn't help. We have our Invoice workflow rules configured based on the Identifying PO field at the Invoice header level to differentiate between the PO and Non-PO backed invoices.