It moves the old kernel modules to the right location for the old kernel to still find them after you've upgraded. When you restart the system to use the new kernel, the old kernel module symlinks are cleaned up.
Fair enough, and my bad, I though the original question was about live upgrading the kernel, but looking at the thread again, they were just asking about the system not breaking. Thanks for putting the effort into explaining!
Is there a way to upgrade your kernel then unplug and replug a USB device without it breaking yet?
Yes. Are you using Arch? Install kernel-modules-hook
Some distros have something similar enabled by default.
Does that update the kernel in-place, or only fix up kernel modules to continue working after the update?
It moves the old kernel modules to the right location for the old kernel to still find them after you've upgraded. When you restart the system to use the new kernel, the old kernel module symlinks are cleaned up.
From what I understand, live kernel patching is only recommended for critical security fixes to server environments where you can't just boot off every user. https://wiki.archlinux.org/title/Kernel_live_patching
Fair enough, and my bad, I though the original question was about live upgrading the kernel, but looking at the thread again, they were just asking about the system not breaking. Thanks for putting the effort into explaining!