Change order for services

For any repair / services type of transaction. First,we take estimation from the vendor and based on estimation user create requisition and PO.  Also dispatch the PO. When services has been done then some of the services (based on estimation) not required and there might be some extra services required and so the price will be affected.

Vendor sends the invoice as updated. but PO is not updated yet and when creating receipts then it will be according to PO. How can we handle this case?

we have checked that we can update PO after dispatch and change order will be created. In this case PO date will be updated as current date. (Can we change it to back date as of original PO Date?)