public class CIMStandardGeneralPlacementProperties : CIMGeneralPlacementProperties, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

{
  // Note: call within QueuedTask.Run()
  {
    //Get the active map's definition - CIMMap.
    var cimMap = MapView.Active.Map.GetDefinition();
    //Get the labeling engine from the map definition
    var cimGeneralPlacement = cimMap.GeneralPlacementProperties;

    if (cimGeneralPlacement is CIMMaplexGeneralPlacementProperties)
    {
      //Current labeling engine is Maplex labeling engine
      //Create a new standard label engine properties
      var cimStandardPlacementProperties = new CIMStandardGeneralPlacementProperties();
      //Set the CIMMap's GeneralPlacementProperties to the new label engine
      cimMap.GeneralPlacementProperties = cimStandardPlacementProperties;
    }
    else
    {
      //Current labeling engine is Standard labeling engine
      //Create a new Maplex label engine properties
      var cimMaplexGeneralPlacementProperties = new CIMMaplexGeneralPlacementProperties();
      //Set the CIMMap's GeneralPlacementProperties to the new label engine
      cimMap.GeneralPlacementProperties = cimMaplexGeneralPlacementProperties;
    }
    //Set the map's definition
    MapView.Active.Map.SetDefinition(cimMap);
  }
}