“When the Joffrey Ballet’s Gerald Arpino steps aside July 1 as head of the troupe he co-founded 50 years ago, it will signal a momentous change for the world-renowned company, based in Chicago for the last 11 years. When the new artistic director is in place, which could be this fall, the Joffrey will be led by someone other than a co-founder for the first time in its half-century history.”