I got the impression from the videos and people's posts here that skyscrapers will be put down by the player rather than through a building upgrading itself.
To me, as a virtual mayor to be, this makes sense. Why? Because in cities large projects usually require special permissions. Perhaps, a middle ground can be found between plopping a skyscraper and it developing by itself. A request to upgrade a building to a skyscraper could appear and the player could choose to approve it or not. Your decision would be made based on design of the building, location and the load it would create on your infrastructure.
To me, as a virtual mayor to be, this makes sense. Why? Because in cities large projects usually require special permissions. Perhaps, a middle ground can be found between plopping a skyscraper and it developing by itself. A request to upgrade a building to a skyscraper could appear and the player could choose to approve it or not. Your decision would be made based on design of the building, location and the load it would create on your infrastructure.