A project has a purchase order number, but it does not appear on an existing invoice. Why?
Changes to the project's purchase order number apply to new invoices created from the project. Existing invoices do not automatically update when the project's purchase order number changes.
For Draft or Unprocessed invoices, you can open the invoice and enter or update the purchase order number directly on the Invoice Overview tab. You do not need to delete and recreate the invoice.
For Finalized invoices, the purchase order number is display-only and cannot be edited directly on the invoice. Check CORE Help Center for details.