A method for modernizing an elevator system, includes using a first and second car for transporting passengers and/or goods between vertically displaced floors of a building, and thereafter removing the first elevator car from use for transporting passengers and/or goods between vertically displaced floors, and thereafter installing a construction time elevator into the lower end of the first hoistway, the traveling zone of the car of the construction time elevator covering only partially the height of the first hoistway; and thereafter using the car of the construction time elevator and the second car simultaneously for transporting passengers and/or goods between vertically displaced floors of the building, during which using the method includes performing construction work in the first hoistway above the traveling zone of the car of the construction time elevator; and thereafter changing the traveling zone of the car of the construction time elevator to extend higher in the hoistway, and thereafter using the car of the construction time elevator and the second car simultaneously for transporting passengers and/or goods between vertically displaced floors, during which using the method comprises performing construction work in the first hoistway above the traveling zone of the car of the construction time elevator.