Is it possible to change the vendor a PO is assigned to after the PO has been saved?
As long as the PO hasn't been received, you can edit the vendor: Open the purchase order, and then click Edit. Delete the existing vendor from the Vendor field, and choose the new vendor, and then click Save.
Is it possible to change the vendor on a PO is after in has been received?
Yes, but you will first have to reverse the receiving:
Go to Purchasing > Stock Receipts and type the Purchase Order.
From the Select Receivings list, click the link to open the receiving you want to edit.
Un-receive everything on this order: In the Quantity Received column, type 0.
You can now edit the vendor as described above.
You can then re-receive the products from this order and proceed as usual.