Advice from Eric Siegmund on keeping a band alive for almost 50 years:
If you seek longevity as a musician, acquire some skill on an instrument other than your voice. Inevitably, the old guys lose the upper registers (or in the case of Gary Lewis, all the registers), but the horn players can rock it until the day they pass to that Great Spit Valve in the Sky. Chicago has done a good job of finding younger replacement vocalists (who are also great instrumentalists) while keeping a core group of four original members.
From what I’m told, Robert Lamm, who wrote a lot of early Chicago tracks, is still singing them with the exception of “25 or 6 to 4,” which he wrote but didn’t sing. (Perfect opportunity for Jason Scheff, who replaced Peter Cetera, who did sing on it.)
As for Gary Lewis, he sings more now than he used to. (Producer Snuff Garrett’s modus operandi back then: have Ron Hicklin do the basic vocal track, add Lewis double-tracked, sweeten with more Hicklin and/or other Playboys.) And there’s always, um, Auto-Tune.