code: func (d *ResourceDiff) GetChange(key string) (interface{}, interface{}) {
  	old, new, _ := d.getChange(key)
  	return old.Value, new.Value
  }
docs: // GetChange gets the change between the state and diff, checking first to see
  // if an overridden diff exists.
  //
  // This implementation differs from ResourceData's in the way that we first get
  // results from the exact levels for the new diff, then from state and diff as
  // per normal.