When you upgrade a SBC from one release to the next using the dnf
upgrade method, it updates all the files in the filesystem in the same way as with a
standard installation - I guess. So it would be fine to use that method on a SBC installed
by arm-image-installer.
But what about the files idbloader.img and u-boot.itb which arm-image-installer copies to
the boot media in a separate dd step? Does the dnf method upgrades those, too, or do I
have to do it manually afterwards? Or is there something else to do?
You need to update it separately, it's a device firmware so it's
treated separately. For the firmwares that reside on the same storage
as the OS (mSD etc) there's a update-uboot tool that works similarly
to arm-image-installer and is part of the same package.