AutoCAD .NET :: How To Set Polyline Start Point

I tried to set Polyline.StartPoint to new position (Point3d):

Polyline.StartPoint = myNewPoint3d;

I have "not implemented" exception.

Also, I tried to do it as:

var seg0 = acEnt.GetLineSegmentAt(0);

seg0.Set(newPoint3d(0, 0, 0), seg0.EndPoint);

It doesn't work also. I can do it as:

acEnt.SetPointAt(0, my2dPoint);

But it's 2D point, not 3d

Of course I can remove polyline and create it again, but. How to set start point correctly for 3d point?