To the best of my knowledge: yes, individual parts are deployable separately, within reason. No, there explicitly no need to deploy the whole thing at once, and especially to shut it down all at once.

Erlang works by message passing and duck typing, so, as long as your interfaces are compatible (backwards or forwards), you can alter the implementation, and evolve the interfaces. Think microservices, but when every function can be a microservice, at an absolutely trivial cost.