Stories have their main characters that the story centers around, along with many secondary and background characters. These other characters vary in their involvement with the MC and the main plot, so the story would be expected to change if one of these characters of less importance was the MC.
It would revolve around a different conflict. Right now, the main plot is about Ellie getting into a cooking school without knowing how to cook because she wants to find her dad. If a background character was the main character, it wouldn’t be about that anymore. All the other kids in Ellie’s group know how to cook, so it would probably revolve around their lives and conflicts.