As of now, it’s unclear why Cassie Lang was recast. In Ant-Man and The Wasp: Qauntumania, Big Little Lies alum Kathryn Newton is going to be taking over for Emma Fuhrmann and will be playing Cassie. It’s unclear how big of a role Cassie will be given, but considering the recast, it’s likely that Cassie will be doing some butt-kicking, too. Emma hasn’t shared why she won’t be coming back to MCU, but we do see that she is involved in several projects this year, per her IMDb page. It’s possible that the actress just had a busy schedule, as she is attending USC.
It’s also possible that Disney/Marvel didn’t ask her to come back, for various reasons we don’t know. Her successor, Kathryn Newton, has been films like Freaky and Pokemon Detective Pikachu, as well as Supernatural, so she has a lot of experience in action-packed projects. In the MCU comics, Kang the Conqueror attacks the Marvel Mansion, where Cassie lives after her dad dies (in the comics!). Cassie, along with other Marvel teen superheroes, like Kate Bishop (who Hailee Steinfeld will be playing in the new Hawkeye Disney+ series) and Iron Clad, defeat Kang the Conqueror. Cassie eventually gets the superhero named “Stature.”
Cassie has the same abilities as her dad, Scott. She can grow or shrink in size, and her abilities are directly tied to her emotions. Like her dad, it takes her a while to get accustomed to her new powers.
Source : https://www.distractify.com/p/why-was-cassie-lang-recast-mcu