I covered what performers there were there for muppets back in the day but here are those who took over for the old performers for various reasons. Where one performer cover 5 Muppets or more, there are performers who split them up.
Kermit the Frog
Famously Kermit was performed by Jim Henson, when he passed away, the family had to make the decision if Kermit would continue and ultimately Steve Whitmire. Jane Henson reflected on the recasting, in a 1990 interview: "Steve Whitmire as Kermit: "When Jim was alive, he said if anything happened to him, Kermit must go on right away. Because of Kermit's significant place, Jim had essentially chosen who he thought could do it. But we don't want to say who it is before the show. The performer needs time. Kermit won't come back so strong at first. Then little by little, he will get his whole personality back."
Eric Jacobson took over in 2001 from Frank Oz. Frank Oz only did voice over for Muppet Treasure Island and Muppets in Space. Eric Jacobson started working with the Muppets in 1994 as a puppeteer on Sesame Street, and rose through the ranks of Sesame puppeteers. In order to keep the characters of Frank Oz - who was focusing more on directing and other pursuits - alive and visible, Jacobson was selected due to his strong puppeteer talent and extremely similar vocal range. Jacobson is currently the principal performer of Fozzie Bear, Miss Piggy, Animal and Sam the Eagle – as Frank Oz had retired as their performer in 2000.
Frank Oz originated the role, Eric Jacobson took over in 2001.
David Goelz still performs Gonzo. He also performs Beauregard, Dr. Bunsen Honeydew, Zoot, Boober Fraggle, Uncle Traveling Matt, Stinky the Skunk, and Rugby Tiger, as well as succeeding the role of Waldorf from Jim Henson.
He was originated by Richard Hunt in 1976 until he passed away in 1991. David Rudman took over in 2008. Though Scooter never made any appearances between 1992 and 1999, a framed photo of him appears on-screen in Muppets Tonight episode 106. Following the death of Richard Hunt, Scooter would not have a consistent performer for 16 years until David Rudman took over.
Janice was originally performed by Fran Brill in The Muppet Show: Sex and Violence pilot in 1975, and in the first set of Muppet Meeting Films. Eren Ozker performed Janice in the first season of The Muppet Show. Richard Hunt took over the character in season two, and gave Janice her familiar Valley girl voice and attitude. David Rudman has since taken over.
Statler and Waldorf
Richard Hunt did Statler from 1976 - 1991, Jerry Nelson originated it in 1975 and took over 1992 - 2003. He died and Steve Whitmire took over 2002 - present. Jim Henson did Waldorf from 1975 - 1990 and Dave Goelz took over 1992 - present.
Rowlf the dog
Jim Henson started it in 1962 to 1990. Bill Baretta took over in 1996. Jim Henson would usually perform the head (and voice) and left hand, while another Muppeteer (including, at various times, Frank Oz, Jerry Juhl, Jerry Nelson, Louise Gold, and Steve Whitmire) performed his right hand.
Richard Hunt started the role, Steve Whitmire took over in 1992.
Jerry Nelson started it and stopped performing in 2003. He died recently. Matt Vogel took over in 2006. Since joining Sesame Street in 1996, Matt Vogel has become most familiar for his work as Caroll Spinney's understudy for Big Bird on Sesame Street, including the "Journey to Ernie" segments, several street scenes and other appearances whenever Spinney is unavailable. He has also taken over some of Jerry Nelson's characters (Robin the frog and Count von count), by the recommendation of Nelson himself, multiple years before Nelson's death.
Grover and Bert
Frank Oz performed Bert and Grover. In 1997, Eric Jacobson started performing Bert on a semi-regular basis, and the next year he added Grover to his list as well; he began to perform both characters full-time with Play with Me Sesame. Today, Jacobson is the principal performer of these two Sesame stars – however Frank Oz occasionally comes in to record new material with them.
Cookie MonsterFrank Oz originated it, David Rudman took over in 2001. Rudman first started working with the Muppets in a summer internship with the Muppet Workshop in 1981, when he was 18 years old. His friend Richard Hunt helped him get a job as a Muppet performer.
ErnieSteve Whitmire took over in 1993 from Jim Henson.