From SPRO, go to SAP Supplier Relationship Management --> Supplier Self Services --> Cross Application Basic Settings --> Settings for Documents --> Define Settings for Follow-On Documents for Inconsistent POs. We checked the box "allow follow-on docs for discrepancy between PO and PCO items" and selected SUS PO Response in the default values box.
Vendors still have to change the date if the delivery date is in the past but are allowed to create the ASN without waiting for the PO to be changed on the SAP side.
Hope this helps.