In most cases, you do not need to update polylines with CONVERT. By default, the PLINETYPE system variable specifies that polylines are updated automatically when you open an older drawing. Polylines may be created in the old format by third-party applications, and they may be contained in an older drawing that was inserted as a block and then exploded.
The following prompts are displayed.
Enter type of objects to convert [Hatch/Polyline/All] <All>: Enter h for hatches, p for polylines, or a for both
Converts all hatches in the drawing.
Converts all polylines in the drawing.
Converts all polylines and hatches in the drawing.