Muppets Tonight was the same and different, actually. The main difference being SS gradually changed over time... The Muppets had to completely reinvent the concept to get back on TV. But in a sense, the same people were gone and new characters had to fill those gaps too.
You know something's up when even the writers don't know what direction they're going in. It took them until the last episode to find their direction. Some of the skits worked beautifully (Once in a Lifetime, Co-Dependence Day, Seinfeld Babies) and some just fell completely flat (BayofPigs Watch, Deep Dish 9)... and some of the best stuff (FTPD, Tubbs of Porksmith, Mr. Callahan) weren't even shown in the US.
As for the characters, this was coming at a time when Kermit had a much more diminished role. They didn't know to what extent they wanted to use him, and so the job of hosting was pawned off to another character. I still think Steve wasn't that comfortable being Kermit yet. The MFS commentary track where Kermit ducks out 2 minutes into the movie, and Rizzo takes over until the credits, and Kermit comes back claiming he was lost in The Phantom Menace commentary was a big clue. Likewise, some characters, Johnny, Sal, Pepe, Bobo, and VanNeuter stuck. Most of the other new characters made for the show ended with the show... though they did bring Andy and Randy back for that Cooking Show.
So any new additions are hit or miss anyway. Some stick, some don't... but they are created out of necessity sometimes.