Changing of the guard in sumo wrestling happens when the new, younger generation of wrestlers out performs the older generation and the yokozuna realize it is time to hang up the mawashi. Sometimes this is noticeable, other times it is so subtle that you don't realize it happened. The last major changing of the guard occurred in 2002/2003 when Asashoryu stormed onto the scene and pushed the older Musashimaru and Takanohana out. Before that it was in 1991 when Takanohana upset the dominate Chiyonofuji and he realized it was time to retire. Taiho, arguable the best Yokozuna in history (we'll see where Hakuho lands though), was an up-and-comer in 1960 and was getting ready to dethrone the great Yokozuna of their time, Wakanohana and Tochinishiki. His rival, Kashiwado, was right beside him and they went on to dominate sumo throughout the 1960s until they too got dethroned in the 1970s. Such is the life in a King of the Hill type sport. Perform or retire.
This scarce 1960 Marumatsu Black and White Bromide (BB-Series) set captures this dynamic time in sumo. I recently picked up the rarer of the two different versions: Type 2 Blue Backs. In them you can see an aging Wakanohana and Asashio. I don't own any of Taiho or Kashiwado in the Blue Backs, but they all exist in this set.....the young getting ready to dethrone the old.