The first two seasons or so (when the series was shot with a single camera) = realistic '50s Midwest America, Chuck Cunningham exists, Fonz is just a cool greaser.

After that (when the series started being shot with three cameras) = new universe where there's no Chuck, and fashion and music is an amalgamation of '50s and '70s/'80s cultures. And the Fonz is a superhuman; similarly, aliens and ghosts and such exist too.

That's as good an explanation as any.

I like version 1 better than version 2. It was always funny seeing the 80's hair styles and Mr. C wearing 80's era glasses. Garry Marshall said he sold out when it went to a studio audience.

The first two seasons or so (when the series was shot with a single camera) = realistic '50s Midwest America, Chuck Cunningham exists, Fonz is just a cool greaser.

After that (when the series started being shot with three cameras) = new universe where there's no Chuck, and fashion and music is an amalgamation of '50s and '70s/'80s cultures. And the Fonz is a superhuman; similarly, aliens and ghosts and such exist too.

That's pretty much it. For all practical purposes the original series was canceled, ABC was not going to renew it, Marshal talked them into give a reworked version of the show a shot. The new version had a new lead actor, the show was an alt universe version of the 1950's, moving the entire decade into the 70's and giving the lead character seemingly supernatural abilities.

Garry did what he had to do. And it worked. From the fall of 1975 until at least 1979 the show was HUGE. The kids at my school had the lunchboxes, trading cards and dolls. The kids loved The Fonz. Did the show get ridiculous? Yes. When they were hanging out with Mork from Ork it was crazy. But the kids loved it. Of course by 1980 it wasn't the coolest show anymore and when Ron left it was done. No one talked about it at school anymore.