The Showrunners Recast Amy's Mom For A Good Reason
CBS didn't make a big deal out of the recast, but The Big Bang Theory showrunner Steven Holland explained the reasoning behind it (via TVLine). When the show initially introduced Mrs. Fowler, Sheldon's future wife was still a newbie in the series, with limited involvement. After eight years, she had become a main cast member and an important player in the narrative, which meant that there was a chance to put more focus on Amy's individual story in The Big Bang Theory.
The sitcom wanted to bring in someone who can play Mrs.
Fowler repeatedly if the narrative called for it and series creator Chuck Lorre already had an existing relationship with Kathy Bates, so they took the opportunity to bring her in. This explanation checks out as after her first physical appearance, Bates' Mrs. Fowler also appeared at the start of The Big Bang Theory's final season.
How Recasting Amy's Mom Changed The Big Bang Theory
Despite running for more than a decade, recasting wasn't the norm on The Big Bang Theory. CBS was able to keep its core cast intact, as well as, its supporting ensemble for years. So, switching actors for Mrs.
Fowler was a rare occurrence. Despite only appearing for only a few times, the recast actually had some impact on the sitcom's narrative. Bates' imposing presence allowed the show to depict the character as a domineering mother and wife, something that was prevalent in Amy and Sheldon's wedding. The Big Bang Theory's final season leaned on it as Mrs. Fowler tried controlling Mrs. Fowler, played by Teller, who recently made a guest appearance on Young Sheldon playing a different character.
Since The Big Bang Theoryabruptly ended after season 12, unfortunately, the sitcom wasn't able to bring Bates' Mrs.
Fowler more. She could have joined the ranks of Laurie Metcalf's Mary Cooper and Christine Baranski's Beverly Hofstadter as the most notable mothers in the series. Given Amy's guest appearances in Young Sheldon, however, perhaps the CBS prequel can also find a way to feature Bates as Sheldon's mother-in-law.
Amy's Mom Wasn't The Only Big Bang Theory Recast
Recasting characters on sitcoms is something that has happened for decades and many of them are pretty memorable from Harriet Winslow on Family Matters to Vivian Banks on The Fresh Prince of Bel-Air to Laurie Forman on That '70s Show.
However, The Big Bang Theory's biggest change is actually one that fans never got to see. In the unaired pilot, Penny was actually Katie, played by Amanda Walsh. It wasn't quite clicking so they recast Penny to Kaley Cuoco and made some changes to the character, which resulted in
The Big Bang Theory officially getting greenlit.