[UPDATE] Later I was able to successfully migrate an amd64 Ubuntu server to Debian, following the same procedure. I had a Ubuntu 22.04.3 on a Raspberry Pi 4. Recently I decided to switch it to Debian 12.5 in-place . Mostly I was following this script: https://github.com/alexmyczko/autoexec.bat/blob/master/config.sys/ubuntu-deluxe It worked surprisingly well, however, the system was eventually broken (unsurprisingly) near the end, when I tried to upgrade the kernel. I tried to fix initramfs by copying dtbs files, which seemed to work. Some references: https://qiita.com/takasan/items/ef93be9e9d3f791eee66 https://bugs.launchpad.net/ubuntu/+source/flash-kernel/+bug/2012750 But the kernel did not load after reboot. Reverting /boot/config.txt didn't help. Then I downloaded a Debian image and replaced only the boot partition of the micro SD card. This time the kernel was able to boot, but it couldn't load the filesystem. I modified cmdline.txt, replaced "root=LABEL=RASPIROOT...
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