If I’m running Debian on my board, but I create a new golden image running Ubuntu and mender-convert it. Can I deploy this new image to all my devices even if it’s completely changing the distro? Also, will files in the /data partition still be preserved and be available after the system update to the new distribution?
In theory yes, since the entire root filesystem is replaced, it should not matter if they are different distributions. In practice there may be some extra steps needed depending on whether the distros have any specific requirements wrt bootloader level integration. It’s difficult to say what issues you will face so I suggest you try it and report back any issues.