Related to other comments, there are some mechanisms that ensure that two versions of all code can remain active (if you chose) in Erlang. No extra effort is re

Related to other comments, there are some mechanisms that ensure that two versions of all code can remain active (if you chose) in Erlang. No extra effort is required. In terms of correctness, I'm not aware of any work that does type checking or serious analysis across version upgrades. Even Erlang's dialyzer will only check each version in isolation. So in practice this means you either test the upgrades with things like continuous integration or even manually, OR do so in very careful and small steps that are easy to reason about locally (very useful for critical applications that can't spare downtime).