During its 11th and final season, Happy Days aired in the 8:30 P.M. timeslot, a change from the 8:00 P.M. timeslot it has occupied during seasons 1 to 10. I would normally see this as a demotion. During the first half of the 1983-84 season, the 8:00 P.M. slot was occupied by a critical and ratings megabomb named Just Our Luck. JOL, upon its cancellation after 11 episodes, was replaced midseason by Foul-Ups, Bleeps and Blunders.

Why was Happy Days demoted to an X:30 P.M. slot after occupying an X:00 P.M. slot for many years? Aside from the fact that Laverne & Shirley, which occupied the Tuesday @ 8:30 P.M. slot during most of its run, had already been cancelled after the previous season (1982-1983)?

Because its ratings, which had been slipping gradually for several years, cratered in the spring of '83 when The A-Team hit the air. Happy Days was no longer competitive enough to lead off the night.

For what it's worth, it did go back to 8:00 for its burn-off episodes in the summer of 1984.

In my opinion, demoting a popular veteran TV show to an X:30 PM slot after having been in an X:00 PM slot for many years is one way to relegate said popular veteran TV show. Other examples of such an occurrence:

Two and a Half Men - After having aired on Mondays at 9:00 PM from Seasons 3 to 9, Season 10 aired on Thursdays at 8:30 P.M., following The Big Bang Theory, and Season 11 aired on Thursdays at 9:30 P.M., following The Crazy Ones. Later that same season, the timeslots for TaaHM and TCO were swapped.

The Facts of Life - After having aired on Wednesdays at 9:00 from Seasons 3 to 6, Season 7 aired on Saturdays at 8:30 PM, following Gimme a Break!, and also to provide a lead-in for the new The Golden Girls. The following season, TFoL was moved to Saturdays at 8:00 PM.

I have another question: What if Happy Days had been cancelled in 1983, and Laverne & Shirley continued for the 1983-84 season instead? L&S would still in the 8:30 PM timeslot, following Just Our Luck.

HD and L&S had similar ratings figures during the 1982-83 season (L&S's is slightly higher, at 17.8, compared to HD's 17.4), yet both sitcoms' ratings fell to new lows after The A-Team premiered at midseason.

I have another question: What if Happy Days had been cancelled in 1983, and Laverne & Shirley continued for the 1983-84 season instead? L&S would still in the 8:30 PM timeslot, following Just Our Luck.

HD and L&S had similar ratings figures during the 1982-83 season (L&S's is slightly higher, at 17.8, compared to HD's 17.4), yet both sitcoms' ratings fell to new lows after The A-Team premiered at midseason.


Wasn't L&S moved to Thursdays in the 80's?

It was moved to Thursdays in the fall of '79 and stayed there for four months, doing significantly less well than Mork and Mindy had in that timeslot. After CBS moved WKRP In Cincinnati to 8PM, ABC saw an opportunity to crush what they apparently considered a vulnerable show and L&S moved to Mondays. It stayed there for a month, getting slaughtered by WKRP and rushed back to Tuesdays, tail between its four legs.