Apologies for the delay in coming back..
About the business process it is some time possible with blanket PO's.. example- You have requirement for a machinery in multiple locations/plants, Initially SC/PO created with some location and proceed with advance payment, No GR/IV created. When you actually received it you want to move that to specific location, So you will have to change the Plant in the PO..
That is where I listed down the scenarios which I came across,
1) PO transferred to ECC- No subsequent document- Allowed
2) PO transferred to ECC- Subsequent document generated- Now allowed
3) PO transferred to ECC- Only Down payment created/reversed - Not allowing- With error- Back End Error:Plant can no longer be changed
In 3rd scenario plant change in PO failing with error- Back End Error:Plant can no longer be changed
BR
Srrimaan