looks like any required changes to work/boot and work/rootfs will need to happen as part of the mender-convert-package script as its only then that the PARTUUID’s will be known once the image partition layout is written out (GPT)
so this looks like it will comprise changes being made to grub.cfg in work/boot as already discussed and also fstab in work/rootfs as it references /boot/efi and /data by device name currently