Its pretty instant. Hitting inflight request still finishes with the old version since the code thats run is already in memory.

I dont think its very different from changing proxy to point to different port.

That's a perfectly reasonable approach, so long as you understand why it's a risky operation and can tolerate the consequences, including customers seeing errors in their browser. If that's OK for your use case, then rock on! If you can't tolerate that, then you have to have switch to a more complex upgrade system, like blue-green deploys behind a load balancer or such. In other words, the deployment method of a Rust or Go or Python or Java app.