Depending on your precise definition of "reboot" it's also possible with newer kernels to run the kexec wrapper around the kexec() system call
and thus load a new kernel without a firmware reboot. (Also available
with the old 2-kernel monte patches and utility).
Of course all user processes are killed through a kexec() ... and all
devices are re-initialized by the new kernel. Thus this is a "reboot"
in that sense of the term.
Combine that with some Mosix and Xen like process checkpointing and suspension and you could very well devise a Linux system that would not
have to be "rebooted" in order to switch in a new kernel.
But it's quibbling to compare the necessity of rebooting into a new kernel
with the infamous reboots required for trivial system settings in earlier versions of MS Windows. I used to have a poster with a genuine appearing MS Windows GUI dialog that read:
You have moved your mouse.
A system reboot is required for this change to take effect.
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