It's been going on last 2 decade. The most significant one was done in 2003-2004. Wimbledon and US Open made a formal announcements on it. US Open added more sands on surface, Wimbledon packed more soil underneath grass layer and changed grass type. They also made the ball touch bigger, heavier and hairier. The Game is much slower. Many players changed style in mid career when this happened in 2003-4. It was not a slow evolution.