The Convert from Bspline option converts a CAD (Bspline) surface into triangulated surface data. You have the option to delete the old CAD surfaces.
To reconvert the faceted surface into a Bspline surface, see Geometry > Create/Modify Surface > Merge/Reapproximate Surfaces.
Note: Converting a faceted surface to a Bspline surface is a difficult operation and requires the boundaries of the faceted surface to be well defined.