It isn't typically news when a jazz group makes a change in personnel. But The Bad Plus isn't a typical jazz group, and its announcement, this time last year, landed like a bombshell. In short: Ethan Iverson, the band's pianist, would be leaving to pursue his own projects. Orrin Evans, an esteemed peer, would be stepping in. For a group that has always stood for musical collectivism — and never accepted any substitutions — this was a shakeup of existential proportions.