I believe it is possible to do container live migration -- and for certain stateful services in particular, may make more sense than "add some nodes, remove some others" as you describe. In a previous product, we built seamless container migration of FreeBSD jails based on a network proxy. It did shut down and start up the process, but as long as the process can be gracefully restarted (which most can!) from the user's perspective it just increases latency slightly.