I wonder if Frank's characters really did start to get recast in 1997 like it says on Muppet Wiki (regarding Eric's characters). I first heard about the occasional recasts (which have become more frequent, in a way Frank's performances could be seen as occasional now) in 2001. Of course I first read about Matt Vogel being the understudy for Big Bird around the same time, and the 40th anniversary book (not to mention a number of interviews) state that he began as Big Bird's understudy in 1996, while his earliest-known vocal performance (on the show itself) was a season 30 episode (where Vogel and Spinney actually alternated the role in the same street story). Though I know that the original purpose for Caroll Spin ney having an understudy was because Sesame Workshop wanted Big Bird to make more live appearances than Spinney wanted to (I wonder if any of these were televised), and they didn't want Big Bird to be silent.
But I haven't read anything official that cites Eric Jacobson performing Bert and Grover earlier than 2001*. Maybe he performed while Frank dubbed the voices, or maybe he was just practicing as the characters then, maybe even doing them in non-speaking appearances. I recently read that in 1999's "CinderElmo", Frank dubbed most/all of his characters lines, with Steve Whitmire and Kevin Clash alternating as Grover (not sure who performed the other characters). It's a wonder Eric didn't perform them in that (was Eric credited for that special?).
Of course the recasts of Big Bird and Frank's characters happened rather quietly, and of course the recasts are awesome (well, Eric's Miss Piggy started out not as good but has improved over time). So I guess there could have been earlier performances that slipped past our radar (or that we just don't remember seeing).
*Actually I read that Eric Jacobson performed Grover in the "ding-a-long" episode from season 30. I remember seeing that episode, and for years I had thought that Grover was silent in that episode, only talking when he was singing along with the others (in other words he didn't have any lines of his own). I brought this up on the talk page and somebody who had the episode mentioned that he did talk. I wonder if it was brief enough for me to miss it the first time I saw it.