In any case in which a section of a highway is deemed by the Commissioner of Highways no longer necessary for the uses of the primary state highway system, or when, in laying out, constructing, or maintaining sections of highways in the primary state highway system, a part of a highway has been or is straightened or the location of a part of it is altered and a section of the highway is deemed by the Commissioner of Highways no longer necessary for the uses of the primary state highway system, the Commissioner of Highways, by and with the approval of the Board, may discontinue such section of the highway as a part of the primary state highway system. In addition, in any case in which an existing crossing by such highway of the lines of a railroad company or a crossing by the lines of a railroad company of such highway is deemed by the Commissioner of Highways no longer necessary as a part of the primary state highway system, the Commissioner of Highways, by and with the approval of the Board, may discontinue such crossing as a part of the primary state highway system. Discontinuance under this section does not constitute an abandonment of such highway as a public highway or such crossing as a public crossing unless the procedure conforms to § 33.2-902. The opening of the new section of highway by the Commissioner of Highways and the entry by the Board upon its minutes of its approval of the discontinuance of the section of the highway or the railroad crossing shall be sufficient to constitute such discontinuance.