R: How to write a function that replaces a function call in with another function call?

E.g. I want to transform the code

mean(x)

to

fn(x)

everytime I see mean in the code.

replace_mean <- function(code) {
  substitute(code, list(mean = fn)) # doesn't work
  substitute(substitute(code), list(mean = fn)) # doesn't work
}

the above two approaches don't work. E.g.

replace_mean(list(mean(y), mean(x)))

What's the best way to do function replacement using NSE in R?

Base R Solutions preferred.