public virtual object GetOriginalValue( 
   int index
)

private static FeatureLayer featureLayer;
private static void DetermineGeometryChange()
{
  featureLayer = MapView.Active?.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
  if (featureLayer == null)
    return;

  QueuedTask.Run(() =>
  {
    //Listen to the RowChangedEvent that occurs when a Row is changed.
    ArcGIS.Desktop.Editing.Events.RowChangedEvent.Subscribe(OnRowChangedEvent2, featureLayer.GetTable());
  });
}
private static void OnRowChangedEvent2(RowChangedEventArgs args)
{
  // RowEvent callbacks are always called on the QueuedTask so there is no need 
  // to wrap your code within a QueuedTask.Run lambda.

  //Get the layer's definition
  var lyrDefn = featureLayer.GetFeatureClass().GetDefinition();
  //Get the shape field of the feature class
  string shapeField = lyrDefn.GetShapeField();
  //Index of the shape field
  var shapeIndex = lyrDefn.FindField(shapeField);
  //Original geometry of the modified row
  var geomOrig = args.Row.GetOriginalValue(shapeIndex) as Geometry;
  //New geometry of the modified row
  var geomNew = args.Row[shapeIndex] as Geometry;
  //Compare the two
  bool shapeChanged = geomOrig.IsEqual(geomNew);
}

//Return the page number that corresponds to the page name field for an index feature

Layout layout = LayoutView.Active.Layout;

//Perform on the worker thread
await QueuedTask.Run(() =>
{
  SpatialMapSeries SMS = layout.MapSeries as SpatialMapSeries;
  Row msRow = SMS.CurrentRow;
  System.Windows.MessageBox.Show(SMS.FindPageNumber(msRow.GetOriginalValue(msRow.FindField("NAME")).ToString()));
});