The Changing of the Guard was because Giancana was gone, Battaglia was dead, Alderisio was dead, Bucciere was not in good Health, Daddano was in jail etc. Accardo pushed for the change. Ricca was not in the greatest health and agreed because he really liked and respected Accardo and he would get his end from him.
The old Grand Ave power base moved to Elmwood Park, not Taylor St. This happened right after 1950.
I would guess that it had something to do with Tony Capezio, who had a bad heart. He died in 1955 while playing golf. I'm not sure if he replaced Accardo as the Grand Ave capo when Accardo moved up, or if Accardo started out under Capezio with Accardo being his personal underboss, then Accardo moved up while Capezio liked where he was at.
