Dear Samantha
I explained to WaiFong before 2 options(swap PO, and using text items for inv. posting.) and it's not worked with PPHL accounting dept.
So I don't have any idea as well.
for me to fix with option 2, you can also has the same PO no. as you mentioned before that you can not swap PO.
the reverse of GR can not be a good ideas because they already posted invoices as well.
Best Regards
JIP