Actor Douglas Emerson asked to be released from his role as Scott Scanlon because he had no interest in acting any more, and in fact he has completely retired from acting since, instead spending several years in the U.S. Air Force and starting a family.
The character of Dylan was only supposed to be on for one or two episodes, because Fox executives weren't sold on either the fictional character or Luke Perry's ability. Aaron Spelling used his own funds to pay Perry's salary during those initial episodes, and the positive audience response led Fox to approve his addition to the regular cast.
Many changes after the pilot episode: The Walsh house changes to a different building (which explains the odd scenery behind James Eckhouse in the re-shot pilot scenes as it matches the Walsh house used in the remaining 10 years of the series, not the house used only in the pilot.) Also, Brandon's car changes from a Chevy Chevette in the pilot to a different model car in later episodes, referred to as "Mondale". Also, Luke Perry does not appear in the series pilot, he is first featured in the second episode. The building for "The Peach Pit" was also changed after a few episodes in season one.
In the first season Donna Martin's mother was named 'Nancy Martin' and played by actress Jordana Capra when she was reintroduced in season 2 she was named 'Felice Martin' and was now played by actress Katherine Cannon.
Although the character Valerie Malone mentions Brenda Walsh as a close family friend, Shannen Doherty and Tiffani Thiessen never once appeared together in a scene on the entire show. Thiessen was only cast as a replacement following the departure of Doherty and this was only explained as a reason for the relationship between the characters by including her on the show.
Dylan's house exterior changed without him moving. The first outside shots showed a white house in a comfy neighborhood. Later it showed at dark wood house set back from the street and down a slight incline.
Brandon's car nicknamed the 'Mondale' was named after Walter Mondale. Mondale was Minnesota's senior U.S. Senator during the late 1970's and early 1980's, and was chosen by the Democratic Party to run for President in 1984, only to suffer a brutal landslide defeat to incumbent Republican President Ronald Reagan.