More likely they will have George in the front carriage next year. He will then be about the same age William was when he started riding in the carriages and as the future King takes precedence over Harry.
This is a clear sign that Harry has been moved down to allow for his nephews and niece to eventually take their place ahead of him.
European monarchies are based on primogeniture and the Sussex family is the second line. It is natural that they should be given lower precedence than the Cambridges. I wouldn't call it a "demotion" as Curryong put it.