However, there is one mistake. Instead of the last line
~ $ bash pack/usr/lib/u-boot/platform_install.sh pack/usr/lib/linux-u-boot-nanopineo2-current /dev/XXX # replace XXX with the actual device
you need to run
~ $ source pack/usr/lib/u-boot/platform_install.sh
~ $ write_uboot_platform pack/usr/lib/linux-u-boot-nanopineo2-current /dev/XXX # replace XXX with the actual device /dev/sdb
In addition it is important to start the partition at block 8192 (at least for my odroid HC2, may be a differnet number on other devices).
This way you can easily write your backups to an sd.
I finally did the update. I used the same procedure as I wrote in the thread I linked in the first post. It worked without problems.
However, in the last step nothing was updated. I think this means that the armbian repos for bookworm point to the same versions as for bullseye (kernel, dts, ...). I guess this will change soon. Only then I will get a new kernel. I hope this will not break the systems. Lets see.