When Jerry Nelson was interviewed for Toughpigs, he said that he stopped performing Snuffy because he was a major character on the street and Nelson was spending a lot of time in England doing Sesame Street (though there is the fact that Richard Hunt performed the puppetry for one season and would have to balance The Muppet Show and Sesame Street, and Michael Earl has said that he lip synced to Jerry Nelson live on set). For years it had been reported that Michael Earl was heard as Snuffy for two seasons, and recently the wiki has noted that it was actually just season 12 where Michael Earl was heard as Snuffy.
I wonder if that makes more sense, that Earl would start doing the voice and puppetry for the season when most of the main classic performers were not really on the set that much for new material, especially street scenes.
Actually, during seasons 10, 11, and 12, a number of Jerry Nelson's more minor street characters were recast to other performers. Both Michael Earl and Brian Meehl performed Sam the Robot in season 10 (Sam hadn't appeared for a season or two), Michael Earl took over as the silent Slimy and as Poco Loco (who had not apepared for two seasons), and Brian Meehl started performing the Mud Man (I should check, but I think Meehl's performances as the character came after Michael Earl left, at that point it seems Jerry Nelson was on the show more often again). I think Richard Hunt also performed Tough Eddie in one episode from this time.
It seems there have been a lot of characters with a consistent performer to have other performers do them for an appearance or two, often really minor characters who made sporatic appearances, but some seem to have been more consistently used as well. Clementine just changed performers a few times, it seems like a few people filled in for Goldilocks when Camille Bonora was doing the character and then others took over after she left, Dolly Parton was first voiced by a singer before Fran Brill took over but occasionally others have filled in, the generic Countess has had a number of performers along with Camille Bonora and Fran Brill. In addition to the ones I mentioned doing Sam, Caroll Spinney performed him at least once. I wonder if the wiki should have a sandbox page for these kinds of one-episode recasts, though I don't know what kind of title to use.