Change PO information even after receiving has started

Occasionally when we order products from our vendors an item could be discontinued or on backorder where they will not send it. The vendor will still send over the products that are available but for items that are discontinued or not available they take them off the order. We have now received a portion of a PO but the rest is still on the way. We are told by the vendor that in order to still get free shipping we need to order something else and send over the revised PO. This is not currently possibly unless we use a PDF editor. Turns out this happens a bit more than usual right now with all the supply chain issues.
Can we enable POs to be edited even after receiving has started?