Originally Posted by shotzette
Per Penny's book, they had originally decided to end the show after S6. However, Cindy Williams changed her mind at the last minute and TPTB used that to leverage most of the cast back.
Most of S7 was shot in a hurry to accommodation Cindy William's visible pregnancy. If you look in the later eps, you can see that she's showing. In the two S8 episodes she was in, she looked like she was ready to pop.
Penny Marshall had lost interest in the series by S7 and did what was only contractually required of her. Michael McKean was shooting/shopping "This is Spinal Tap" and making/promoting "Young Doctor's in Love". He's said in multiple interviews that the show had been very lenient with him regarding his other projects.
Betty Garrett had already committed to a play on Broadway when she was told that the show was going to run another year, hence Edna's "divorcing" Frank.
Supposedly, the 7th season of the show was intended to be the last so much of the cast had made plans for other projects. At the last minute, it was decided to go on for one more season so that's why David L. Lander is a regular but Michael McKean (Lenny) only made a few guest appearances. Betty Garrett
was doing a Broadway show so they wrote her character had run off and left Laverne's dad. Apparently the episode where Laverne finds the note that Shirley has left was supposed to be the one dealing with Edna leaving Frank but when Williams left they rewrote it to deal with the more pressing void. The Edna-Frank situation was ignored until the second half of the season.