In the early episodes, Kelly was just a street-smart juvenile delinquent who had no interest in school. But at some point she became a *beep* bimbo. When did that change happen? Was it a gradual thing or did they do some sort of character re-tooling between seasons?

Well Kelly did use her looks to get pretty much what wanted until Season 5 when she graduated from Polk High then she got a little smarter with working as the Verminator and going to Larry Storch's acting school.

Kelly changed from tough girl to vamp to ditz and seemed to shed IQ with each passing season.

Lazy writing is what it adds up to.

After the first season and then again at the time when Steve left.

The season where she was the dumbest was written off as a bad nightmare of Al's, when Katy Segal had her miscarriage.

After that the show went to hell. I didnt even realize that the show was still going after I stopped watching it at that point hat as Fox burried it in the schedule.

There was a episode, don't remember which, where the whole family is in the car, where they were much younger, and kelly is reading, she bumps her head and puts the book away.

The show makers hinted that was the moment she turned from smart to erm less smart.

If it ain't broke, don't fix it. But those stupid writes broke and ruined it.

Definitely gradual. Especially as the whole show went from an edgy satire into a shallow buffoonery of its former self (starting around around season 3 as the audiences loved the increasingly bawdy and outrageous humor, but really going off its rails by season 7 and only getting worse from there... mostly. I still liked a lot of the rebounding final/11th season, but that's another argument in of itself... nothing compares to the show's first four years.

And the more Christina Applegate showed her genius at acting, the more it was exploited. (I've nothing but awe and respect for her abilities, it takes a TON of talent and i'm surprised she didn't get more diverse roles in the industry.)