Undo a Cap Part

Insert a symbol from the Explorer pane that is similar to the one you want to create.

Click on the Cap Part toolbar.
When you undo a Cap Part, it will strip all attributes, but not the nodes from the object.

Select the part on the drawing.

Press Enter to confirm your selection.

Modify the symbol by using the AutoCAD Stretch command, or erasing unwanted lines.
Now, with the symbol drawn or modified, you must make it smart. See Create a new Cap Part.