If you want to create a credit note for a vendor, create a new purchase order for that vendor and then click on the credit note button, select the invoice you want to credit and the lines of the original purchase order are added to your new purchase order with inverted quantities. In this way you can credit a previously invoiced purchase order.
But if you propose this to client we cannot justify that for reversing the invoice of a purchase order, we have to create another purchase order. The question is not about what is the process now and how to use it, the qustion is what should be the correct process and how it should be done.