Ok. I can see that. So if a building changes with parking. I would update the 2d first. then in the 3d, do a ncopy then convert polyline to feature line and off you go. Makes sense to me.
NO, you do not do an NCOPY....either edit existing feature lines if it is a simple change OR
use the [XREF] option when you use create feature line from object command.
Command: _AeccCreateFeatureLines
Select lines, arcs, polylines or 3d polylines to convert to feature lines or [Xref]: *Cancel*
This option is available during many C3D commands.