A client of ours asked this question and after speaking to Sage 50 support team, they said it isn't possible and is intended by design in other ways but to submit this to the helpdesk.
Basically, the client wants to be able to update warehouses on multiple lines of a purchase order as currently creating copy purchase orders does not update the warehouses. So, new idea is to be able to update warehouses on multiple lines, currently when creating a copy layout, it allows you to change the default warehouse but the corresponding lines do not get updated.