PO corrections and encumbrances

I have two questions. Why are funds still encumbered after a PO has been closed? How do I correct a PO that has already been paid-not just the funding? There are three errors I have come accross that need to be corrected. In one instance, the incorrect PO was paid, so it will involve correcting two POs. Secondly, an expense line was paid instead of a PO, so I would like to correct the PO to reflect payment from it. Last, a payment was made from a PO, but it should have been an expense line. thanks