Helper function - generates a SetItemField which tells Exchange you want to overwrite the contents of a field.

def update_property_node(node_to_insert, field_uri):
  """ Helper function - generates a SetItemField which tells Exchange you want to overwrite the contents of a field."""
  root = T.SetItemField(
            T.FieldURI(FieldURI=field_uri),
            T.CalendarItem(node_to_insert)
        )
  return root