Document document = commandData.Application.ActiveUIDocument.Document;

//create analytical panel
AnalyticalPanel analyticalPanel = CreateAnalyticalPanel.CreateAMPanel(document);
if (analyticalPanel != null)
{
   // Start a sketch edit scope
   SketchEditScope sketchEditScope = new SketchEditScope(document, "Replace line with an arc");
   sketchEditScope.StartWithNewSketch(analyticalPanel.Id);

   using (Transaction transaction = new Transaction(document, "Modify sketch"))
   {
      transaction.Start();

      //replace a boundary line with an arc
      Line line = null;
      Sketch sketch = document.GetElement(analyticalPanel.SketchId) as Sketch;
      if (sketch != null)
      {
         foreach (CurveArray curveArray in sketch.Profile)
         {
            foreach (Curve curve in curveArray)
            {
               line = curve as Line;
               if (line != null)
               {
                  break;
               }
            }
            if (line != null)
            {
               break;
            }
         }
      }

      // Create arc
      XYZ normal = line.Direction.CrossProduct(XYZ.BasisZ).Normalize().Negate();
      XYZ middle = line.GetEndPoint(0).Add(line.Direction.Multiply(line.Length / 2));
      Curve arc = Arc.Create(line.GetEndPoint(0), line.GetEndPoint(1), middle.Add(normal.Multiply(20)));

      // Remove element referenced by the found line. 
      document.Delete(line.Reference.ElementId);

      // Model curve creation automatically puts the curve into the sketch, if sketch edit scope is running.
      document.Create.NewModelCurve(arc, sketch.SketchPlane);

      transaction.Commit();
   }

   sketchEditScope.Commit(new FailurePreproccessor());

}

return Result.Succeeded;