Supplier Site merged with invoice only but now needs to merge PO as well.

I have a general question about merger:

I have copied a site # 99 of a vendor 'ABC' to a vendor called XYZ with the option of invoices only but not PO. System put an end date on the supplier ABC's site # 99 ( as it has become a new site of XYZ). User now wants to merge PO as well. System does not allow to remove the end date. How do we merge PO now?

Any suggestion is highly appreciated.

Thanks