2020.
Most people I know stopped with 2nd Ed, and kind of ignored the whole CyberGeneration. Sure, Arasaka set off a nuke in the middle of the city during the 4th Corp War, but that doesn't physically move Heywood up and above Pacifica..
Edit: Don't get me wrong, it's not a complaint, I'm just vaguely curious on the reasons for things like this. When you've had a focus on getting the lore right, there's got to be some sort of technical reasons for changing the layout of the city.