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 OK'ing his addition to the regular cast.
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.
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.
The "gang" were juniors in season one, which meant on an accurate timeline they would have graduated at the end of season two. Instead they were juniors twice and graduated in season three's finale. In addition to re-booting the rest of the cast's timelines, the show first changed David Silver from a sophomore to a junior, and then had him taking a "double set of classes" so he could then graduate with the rest of his friends. The producers said later that they were aware of the blatant unexplained changes but made them anyway because they wanted the series to run in high school as long as possible, since no show had ever gone from high school to college and remained successful (as 90210 would in fact pull off between 1993 and 1997).
Potomac, Maryland, Darren Star's hometown, was the inspiration for the show, and was originally going to be named "Potomac, 20854". However, it was believed that more people could associate with and recognize Beverly Hills as opposed to Potomac.
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.
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.