public enum MaplexGraticuleAlignmentType : System.Enum, System.IComparable, System.IConvertible, System.IFormattable  

//Note: call within QueuedTask.Run()

//Get the layer's definition
var lyrDefn = featureLayer.GetDefinition() as CIMFeatureLayer;
//Get the label classes - we need the first one
var listLabelClasses = lyrDefn.LabelClasses.ToList();
var theLabelClass = listLabelClasses.FirstOrDefault();
//Modify label Orientation                 
theLabelClass.MaplexLabelPlacementProperties.GraticuleAlignment = true;
theLabelClass.MaplexLabelPlacementProperties.GraticuleAlignmentType = MaplexGraticuleAlignmentType.Curved;

lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition