Another thing about the timeline is that from the 1980s until the very late 1990s, it was rare for Jim or Frank's characters to appear in street scenes, the usual exception being in a movie, special, or book. I noticed this even as a kid (one who not only didn't know they were puppets, but who didn't know they were performed by people who had become very busy with other projects by then). Of course most of Jim's characters except for Ernie and to a lesser extent Kermit seem like the kind who might be out-of-place in street scenes most of the time (can you imagine Bad Bart or Sinister Sam coming to the street for some reason?). I was amazed to see Ernie, Bert, and Cookie Monster at Luis and Maria's wedding (even though they didn't talk) as well as when I saw the Best Friends Blues clip.
And then they started coming back to street scenes at the end of the 1990s, especially season 30. Actually that year while it was fairly common to see Ernie, Bert, Cookie, and Grover on the street, most of them were usually in non-speaking cameos. But then the next season it was common to see Ernie in street scenes again, and then the next Frank's characters were starting to appear on the street frequently, due to the substitute performers.
I first remember hearing about Frank's characters having alternate performers in season 32 (though David Rudman and Eric Jacobson sometimes say that they just consider it filling in for him, I think I've performed them long enough and frequently enough to be considered their main performers now, especially since Frank hasn't performed Cookie or Bert in at least five years), but Muppet Wiki says that Eric started performing them in 1997. Muppet Wiki doesn't cite a source for that, so I don't know where 1997 comes from. Maybe they were just preparing to take over then, maybe Frank dubbed their voices a few times when they performed, maybe they were just performing them in non-speaking background appearances. I'd like to see an interview with Eric Jacobson where he clears up when he started performing them (he doesn't get interviewed much).
I've read that Eric Jacobson performed Grover in the Ding-a-Long episode from season 30. I remember seeing that one and don't remember Grover getting any lines of his own, but others on Muppet Wiki say that he did (well, it's their video copies against my memory). In fact Cookie Monster did talk in some street scenes in the season 30 episode where Big Bird gets C is for Cookie stuck in his head. I wonder if Frank performed him there or if somebody else did.