The said courts respectively, shall also have power in the manner aforesaid, to change, or supply by a new road, the route of any state road which may be laid out by direction of any act of assembly, within their respective counties, and thereupon to vacate so much of such state road as shall be supplied: Provided, That no change shall be allowed in any such road, which shall make the same of a greater ascent or descent than five degrees from a horizontal line.