Comment # 16 on bug 1233298 from Ralf Habacker

(In reply to Takashi Iwai from comment #14)

> And you don't have to remove the previous kernel at all.  The kernel
> packages are multi-versioned, and you can choose the right one to boot via
> GRUB menu.

On the live CD a boot menu looks as

menuentry "openSUSE Leap 15.6 KDE Live" --class os --unrestricted {
    set gfxpayload=keep
    echo Loading kernel...
    linux ($root)/boot/x86_64/loader/linux ${extra_cmdline} ${isoboot}
splash=silent quiet systemd.show_status=yes 
root=live:CDLABEL=openSUSE_Leap_15.6_KDE_Live rd.live.image
rd.live.overlay.persistent rd.live.overlay.cowfs=ext4
    echo Loading initrd...
    initrd ($root)/boot/x86_64/loader/initrd
}

The file system is organized by overlays, one read-only and one writable (see
https://documentation.suse.com/appliance/kiwi-9/html/kiwi/building-types.html#hybrid-iso
for details)
The new kernel is installed in the writable overlay, which is not accessible
when booting Grub. Grub starts a base Linux kernel from
($root)/boot/x86_64/loader/linux, which mounts the overlays and somehow
transfers control to the installed kernel, which is not the new kernel,
although vmlinuz points to it. The configuration to select the new kernel is
currently unknown to me.