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);