There is a movie by the name Ship of Theseus which explores the idea in the context of organ donation and transplantation.

I loved the movie. There is one actor by the name of Neeraj Kabi who lost a lot of weight for his role as a monk.

Here is the trailer: https://youtu.be/tzw1ezTTVEY?si=NfsG9sY1R1JjmbVV