Is there any way purchase order headers can be edited (supplier, supplier purchasing site) after in

Is there any way  purchase order headers can be edited (supplier, supplier purchasing site) after initial PO approval.

We had a specific issue with supplier purchasing and pay site not matching – due to VAT change during the middle of purchase order execution.  It appeared that it was going to take several hours from several people to “unwind” customer sales orders, inventory, receipt returns, etc. to fix the issue and get the vendor paid.  However, we were able to fix the issue by changing the pay site in the A/P module . 

On other occasions, the incorrect supplier record or site is chosen during requisition or PO creation, which if not corrected prior to approval could potentially take considerable time to fix.