This is how I think things change:
Every day, you exit the same train station. There's a guy on the corner. He has a sandwichboard reading "GRAVITY IS A CHOICE" and a fistful of folded leaflets. You go to work.
One day, you come out of the train station, and the guy is two stories up one of the buildings on the corner. He's standing on the side of the building as if on the ground, at right angles to everyone else. Someone makes a joke about his commitment. You go to work.
The following day, the guy is gone, but a sign has appeared. It reads, "Please keep on the sidewalk." It doesn't say, "No Walking On The Side Of Buildings." That would be crazy. You go to work.
It all comes to a head later that week, when the quarterly earnings call for a major corporation is interrupted by a skateboarder gliding across the boardroom window in a stately arc, making only a quiet clickCLICKclickCLICK going over the gaps between the big panes. This event comes to be described as a "security incident."
The problem is, it's useful space, you know? And it doesn't exactly belong to anyone. There's some room for self-interested interpretation. The people who are most comfortable with self-interested interpretations start to assert exclusive control of it. This works out pretty well for them in the short, medium, and long terms.
The ground remains the prime surface for most people. Travel between adjacent buildings is something no one has quite figured out, so most trips include the street. And probably 70% of people never go sideways at all. But walking sideways has become a common sight; the big delivery companies have started requiring it in their job descriptions.
They say that if you haven't done it before, you should stand against a wall and feel yourself lying down. Start to bend at the waist. You should feel two forces. One is gravity. The other is no. It is the impossibility of this being true, the number of certain things that this will make not-certain. It is the catastrophe that will retrospectively have befallen you; your having been one way for such a long time and now being another. You can't overcome no, exactly. No has a point. You lose something in this, whatever you do.