Common Lisp has a bunch of features designed to enable migrating the system. e.g. update-instance-for-redefined-class ( https://www.lispworks.com/documentation/HyperSpec/Body/f_upd... ) lets you write code to update instance data between class versions when a class definition is reloaded.

It turns out, though, that making hot-code reloading work well is mainly a question of how you design your system: designing for hot code reloading isn't all that hard for 90% of cases once you figure out the relevant techniques.

We do this in q/kdb+ systems often for patches. An important thing about these languages is that this kind of workflow is part of the core for solving problems. So when you are building a system one of the aspects of its design will always allow for this update method. Then when you push a patch you both know the impact of the change (because you've tested the exact same steps in a dev/QA/UAT/Beta environment) and the work required to do it safely.

Major releases do go through a full shutdown and release cycle though.