I have 'Assigned To' field on a list. On some items the person in the 'Assigned To' has left. How would I change it? I set up a short workflow to update the field for any items which had the person who'd left, but it won't allow me to run the workflow on these items manually. When I try to change the items it says I don't have permissions. When I've tried giving myself permissions before on different lists it hasn't worked.