Vicky,
It is possible to have them switch by adjusting styes on the zoom levels of the shapefile and by adjusting the upper and lower threshold on the WorldMapKitLayer. Just to be sure are you going to use the WorldMapKitLayer or the Overlay in the desktop edition? Here is the rough code.
ShapeFileFeatureLayer shapeFileFeatureLayer = new ShapeFileFeatureLayer();
WorldMapKitLayer worldMapKitLayer = new WorldMapKitLayer();
worldMapKitLayer.UpperThreshold = shapeFileFeatureLayer.ZoomLevelSet.ZoomLevel08.Scale;
worldMapKitLayer.LowerThreshold = shapeFileFeatureLayer.ZoomLevelSet.ZoomLevel20.Scale;
shapeFileFeatureLayer.ZoomLevelSet.ZoomLevel01.DefaultLineStyle.CenterPen.Color = GeoColor.StandardColors.Red;
shapeFileFeatureLayer.ZoomLevelSet.ZoomLevel01.ApplyUntilZoomLevel = ApplyUntilZoomLevel.Level07;
David