So there's this magical limit of ~21 million bitcoins that can theoretically exist at max. Many assumptions about the future of bitcoin are based on this number.

How easy / hard would it be to change this limit? It seems, this function controls how many coins are given to miners for solving a block. If developers decided to change this function, the 21 million number would change too, right? Or is the 21 million number somehow coded deeper in the system?