Once a PO line has been partially processed the VAT Code is locked on those lines (regardless of whether the line has been fully delivered, partially delivered or Invoiced, etc)
The only way to edit the PO's VAT Code for the lines would be:
Set the GBP Price to be zero for the locked line and then Enter a new line for the same quantity and original GBP Price, then entering the correct VAT Code for that line.
Closing the locked lines using Close Lines and then entering the new lines for the same quantity and original GBP Price, then entering the correct VAT Code for that line(s).
Closing the entire PO (or lines) and entering a standalone Purchase Ledger Invoice for the value of the lines.