Hi,
To convert a Spline into a Polyline, just use the AutoCAD .NET API provided Spline.ToPolyline() method (have a look at the docs for the different behaviours according to the PLINETYPE and PLINECONVERTMODE settings).
To convert an ellipse (or an elliptical arc) into a Polyline, you can use the Ellipse.ToPolyline() extension method provided in the GeometryExtensions assembly.
In both case the methods return an instance of Polyline (more accurately a Curve object with Spline.ToPolyline() method). If you want to replace the source object with the newly created one, you have to add to the source object owner and erase the source object.