The LENGTHEN command changes the length of lines, polylines, arcs, and splines and the included angle of arcs.
In AutoCAD MEP 2024 toolset, LENGTHEN does not work on duct, pipe, cable tray, or conduit segments. You can however use Lengthen grips to modify the length of segments. Depending on the editing mode you select, the grips move or lengthen connected objects to maintain their connectivity.