I process a negative qty receipt using the old vendor name; change the purchase order to the new vendor code/name and re-receive the items with the new vendor name.
 
In AP-C you shall have to process 2 invoices
 
One for $0 against that po for the wrong vendor name, and one for the correct vendor name and to process your invoice. (you may have to search by vendor code to find the receipt in AP-C, because now there are two vendor codes associated with one purchase order)