I was a little confused by the title as it implied a relationship between a process's message box and some module which is not the case.

The article does at least demonstrate that processes can transition between two versions of the same code w/o resetting state, which is, at its core, the very thing that makes code upgrades remotely practical.

Other comments mention some more sophisticated machinery like release upgrades. Erlang also has many code upgrade options baked into the OTP as well. I make use of many of these features both during development but also in production with some careful review. I'm always disappointed when going back to a system that has to "reboot" itself after getting used to hot upgrades and distributed erlang (version discrepancies in a cluster can present a similar problem if you don't want to pause your system).