You can't actually change the perspective in 2D. That's silly.
What I'm saying is that with these continuous perspectives, every frame you can possibly take out of the big picture is going to be of a sound perspective. This means that when you scroll it slow enough (as in a background), the visible part is going to seem real.
I don't see why you would see buildings at different angles? Whatever part of the image you take that is maximally one segment wide is going to be in correct perspective, with correct angles. The image breaks down only when extending it too far upwards or when moving it too fast (with a fully rendered scene, it would make you dizzy, as if looking at those annoying fish-eye skate movies, but different).
In summary, the image as a whole might not make sense, but any segment will. It's still faked, but it makes sense and looks the part...