A simple example of an Erlang server that can dynamically change the code to process new requests.

hot_change.erl

-module(hot_change).
-export([start/0, send/1, update_code/1]).

start() -> 
    F = fun(Str) -> string:lowercase(Str) end,
    register(hot_change_server, spawn(fun () -> loop(F) end)).

loop(F) ->
    receive
        {request, Pid, Q} ->
            Res = F(Q),
            Pid ! Res,
            loop(F);
        {update, F1} ->
            loop(F1)
    end.

send(Q) ->
    hot_change_server ! {request, self(), Q},
    receive
        Res -> Res
    end.

update_code(F) -> hot_change_server ! {update, F}, ok.

A possible executing inside erl

1> c(hot_change).
{ok,hot_change}
2> hot_change:start().
true
3> hot_change:send("Hello World!").
"hello world!"
4> hot_change:update_code(fun(Str) -> string:uppercase(Str) end).
ok
5> hot_change:send("Hello World!").
"HELLO WORLD!"