The new SPLINE objects introduced in R13 are nice, but there are limits
on what you can do with them. So conversion to a polyline is necessary.
Carefully erase all the geometry except for the splines, then save the
drawing in R12 DWG (A2K users use R12 DXF) format. Then open the
original drawing, erase the splines and insert the R12 file. The spline
will now have become polylines. If the splines had differing elevations
on the vertices, the resulting polyline will be a 3D polyline. ToolPac
users will find a routine in the utility section to convert them in
place in the current drawing.