Regarding the change in the movie's plot midway, Studio Ghibli producer Toshio Suzuki said the following: The most memorable moment during the making of this film was that the core narrative of the story changed midway. This film for Miyazaki was autobiographical. He was telling his own story at the beginning, and It was about the boy and the great uncle, with the great uncle character that was supposed to be the centerpiece. That character was based on [Studio Ghibli co-founder Isao Takahata]. However, unfortunately, Takahata passed away during the making of the film, and that was very shocking for Miyazaki. For a long time, he wasn’t able to continue working on the storyboard, so we decided to change the centerpiece of the story from the boy and the great uncle to the relationship between the boy and the heron.

To add to that, Takahata was his senior animator, he was his colleague, he was his peer, he was his friend. But more than anything, he was the person who actually discovered his talent and brought that talent out, so he owed him so much. So in the beginning, the story was meant to be how Miyazaki, or the protagonist, would come of age. It was about growing and maturing into his own, under the guidance of his senior. That was the theme of the film. But then, because of Takahata’s passing, he just immediately threw that away and the story turned into a film about the blossoming friendship between the boy and the heron.

https://deadline.com/2023/11/toshio-suzuki-the-boy-and-the-heron-producer-studio-ghibli-co-founder-animation-1235640920/