The release notes for each version has the detailed instructions for how to upgrade from the previous version. They very strongly emphasize backing up the entire system, because the upgrade process is absolutely not guaranteed to leave the computer in a state that works correctly or boots at all, and there is no way to undo it.
That's what's different about NixOS: if the new configuration doesn't work properly, you can just choose the previous configuration in the bootloader menu. It's as straightforward and foolproof as switching to another branch in a Git repository. The whole point of NixOS is that you can have any number of system configurations in the Nix store and switch between them in a way that's atomic in the transactional sense.
If you know how Nix works, you can feel safe turning off the power supply at any point during a full system upgrade. You'll never end up in a state where half of the packages are upgraded. It's either upgraded or not. As far as I know there's no other distribution that works like this other than GuixSD.
"DANGER!!! DANGER!!! DANGER!!! DANGER!!! DANGER!!!
DANGER!!! DANGER!!! DANGER!!! DANGER!!! DANGER!!!"
The release notes for each version has the detailed instructions for how to upgrade from the previous version. They very strongly emphasize backing up the entire system, because the upgrade process is absolutely not guaranteed to leave the computer in a state that works correctly or boots at all, and there is no way to undo it.
That's what's different about NixOS: if the new configuration doesn't work properly, you can just choose the previous configuration in the bootloader menu. It's as straightforward and foolproof as switching to another branch in a Git repository. The whole point of NixOS is that you can have any number of system configurations in the Nix store and switch between them in a way that's atomic in the transactional sense.
If you know how Nix works, you can feel safe turning off the power supply at any point during a full system upgrade. You'll never end up in a state where half of the packages are upgraded. It's either upgraded or not. As far as I know there's no other distribution that works like this other than GuixSD.