Order stopOrder;
stopOrder.StopPriceChanged = stopOrder.StopPrice - 4 * stopOrder.Instrument.MasterInstrument.TickSize;

private void OnExecutionUpdate(object sender, ExecutionEventArgs e)
{
  // Change the stop order if an execution results in a long position
  if(e.MarketPosition == MarketPosition.Long)
      myAccount.Change(new[] { stopOrder });
}