SketchEditScope

public void ReplaceBoundaryLine(Document document)
{
   FilteredElementCollector floorCollector = new FilteredElementCollector(document)
      .WhereElementIsNotElementType()
      .OfCategory(BuiltInCategory.OST_Floors).OfClass(typeof(Floor));

   Floor floor = floorCollector.FirstOrDefault() as Floor;
   if (floor == null)
   {
      TaskDialog.Show("Error", "Document does not contain a floor.");
      return;
   }

   Sketch sketch = document.GetElement(floor.SketchId) as Sketch;

   Line line = null;
   foreach (CurveArray curveArray in sketch.Profile)
   {
      foreach (Curve curve in curveArray)
      {
         line = curve as Line;
         if (line != null)
         {
            break;
         }
      }
      if (line != null)
      {
         break;
      }
   }

   if (line == null)
   {
      TaskDialog.Show("Error", "Sketch does not contain a straight line.");
      return;
   }

   // Start a sketch edit scope
   SketchEditScope sketchEditScope = new SketchEditScope(document, "Replace line with an arc");
   sketchEditScope.Start(sketch.Id);

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

      // 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 FailuresPreprocessor());
}