They never said that.

It was a bit more subtle. I think the point was, that you don't have to restart your machine to begin the installation.

If your kernel is updated, you need to reload the kernel in some manner. Linux has kexec, but requires you to shut down services and reload the kernel nonetheless.

But you can certainly start the upgrade. You can perform the majority of it, in fact.

you're right, i misunderstood it.

Way back (2000? 2001?) when I was solely a windows guy, a friend was helping me install a linux box (showeq...). I can't remember the distro he used, but I do remember starting from an old windows box and installing linux, installing the applications and so forth. Two hours into playing with the new system, my friend asked if I'd noticed how many times we'd rebooted during the installation and setup - none. We installed from scratch to a functioning system without ever rebooting.

Of course, you want a reboot with any major upgrade to make sure that your system works from a cold start, but it was very cool at the time to have a working system from scratch with no reboot...

What would the point of that be, really? The one time every two years you upgrade your OS, you don't have to reboot, saving 15 seconds a year?