Recursive Adaptation: The late 90s/early 2000s Sabrina animated series (and its versions of the cast, etc.) was adapted back into the comics, in the context of being flashback stories to Sabrina's junior high years.
Actor Allusion: When Tim, played by George Wendt of Cheers fame, walks into Hilda's coffee shop in a later episode, everyone raises their mugs and greets him, "Tim!".
Aunt Irma is played by Barbara Eden, who also played in another supernatural sitcom. Her magical gesture in both? Crossing her arms.
Robin Ricker plays Cousin Marigold, the mother of a bratty witch daughter. Around the same time she also starred in the Buffy the Vampire Slayer episode "Witch", where she also played the mother of a witch daughter or so we think as the episode goes on. Buffy even says during the episode "she's our Sabrina".
Still another from Cheers: Diane is a lovely-yet-wicked witch whose cottage is on top of a giant beanstalk. And she's brunette—perhaps in an inside-joke Light Feminine Dark Feminine contrast to Diane, or...something.
The Other Darrin: Similarly to M*A*S*H, very few actors carried over from the original Pilot Movie to the series proper. Other than Melissa Joan Hart, only Michelle Beaudoin, who played Marni in the movie and Sabrina's (first) best friend, Jenny, in the series, appeared in both of them. Hilda, Zelda, Harvey, and the voice of Salem were all re-cast. Libby does not appear in the pilot; a character named Katie fills a similar role. There's also a character named Seth in the movie who acts as Wrong Guy First; he has no analogue in the series, as Sabrina is hot for Harvey (which is more or less requited) from the outset.
A more conventional example, during the show's run: the two actors who played Sabrina's father not only looked totally different but the replacement was visibly years older.
Cousin Marigold is another example, first played by Robin Ricker and later Hallie Todd.
Real-Life Relative: Recurring character Amanda (Sabrina's evil cousin) was played by Melissa's little sister, Emily Hart. Who, as noted above, ended up playing Sabrina herself in the cartoon.