Splines to Polylines
Download VLX from page below and use APPLOAD in AutoCAD to load it, then SP2PL to run it:
Polylines to splines
Copy / paste the script from the page linked below into a text file, then save as pl2sp.lsp
APPLOAD in AutoCAD, then PL2SP to run it
Here is a routine that allows you to turn a “Fit Curve” Polyline into a Spline...
AutoLISP: Polyline to Spline | AutoCAD Tips: