Totally going from memory here, but I seem to recall that the kid that played Larry, moved to the mid-west or east coast, effectively ending his role on the show. Don’t remember what happened to Judy. Gilbert (Stephen Talbot, son of actor Lyle Talbot) stepped in where Larry left off. It seemed to me that Gilbert was an even bigger rat than Larry, and managed to get Beaver into even worse trouble.
I also recall that the later episodes centered more around Wally and his friends. It seemed that puberty was an awkward stage in a young Mr Mather's life, and he had somewhat morphed into an awkward and goofy acting teen. Almost overnight, it seemed that his natural ability to act and recite cute kid lines, in a way that cute kids do, was gone. That was about the time that you noticed that the episodes shifted more over to Wally.
Bit of Trivia: At one point, Ken Osmond (Eddie Haskell) and porn star John Holmes, were thought to be one in the same person (Which may or may not have been a good thing, depending on Mr Osmond’s success with the ladies 😜 ). I once saw some footage of Mr Holmes, and I can totally see how they could have been mistaken for one another. At that time, they looked very much the same, and he (Holmes) even had very similar sounding voice, with the same wise cracking “Eddie Haskell like” demeanor.