You can do this with (or without) any proxy too. Iptables (due to conntrack) can make decisions about the first packet in a connection, then save the result. That means you can match incoming traffic on --state NEW and route to a specific local port. Changing one iptables entry is atomic, so you can swap the version seamlessly this way.
I think the idea is worth mentioning because people often think it's hard to achieve / something special. But there's a hundred ways to do seamless version swap on a host.
