You can convert all, or a selection of, the closed polylines in an existing drawing to polygon objects. When you convert a closed polyline, the original polyline is erased and is replaced with a polygon object.
To convert polylines to polygons
Each closed polyline in the selection set is converted. If the polyline belongs to a group, only the first (outermost) polyline is converted. Other polylines in the group are copied into the polygon as additional boundaries and the polygon is rebalanced.