Following-up:  I would recommend the better solution in post #4, but this solution specifically answers the original question:
Steps required to upgrade the DEVUAN desktop-live .ISO (without persistence)

• meaning that it is a good method to choose if you plan to do this once or rarely.

• perhaps not so good if you are doing much customization

For lack of patience at this moment, I'll be a bit terse.

• boot the DEVUAN desktop-live .ISO and select "TO RAM" from the boot menu

• sudo apt -y remove live-tools

• sudo nano /etc/kernel/post{inst,rm}.d/zz-update-grub
  # in the space after the if-then clause containing "systemd-detect-virt", add this line (once for each file):
  if (grep --quiet QEMU /proc/cpuinfo); then exit 0; fi

  (attribution to this post: https://dev1galaxy.org/viewtopic.php?pid=62626#p62626 )

• sudo apt -y remove grub-efi-amd64
  sudo apt -y update
  sudo apt -y upgrade
  sudo apt -y autoremove
  sudo apt install live-tools

  note: grub efi support was just removed.  implications thereof?

• get your storage-media ready for use

  sudo mkdir /mnt/space
  # replace Zda below with your own empty storage media or partition, e.g. sda3, vda1, sdc
  sudo mkfs.ext4 /dev/Zda # only do this if you need to format the space
  sudo mount /dev/Zda /mnt/space
  sudo mkdir /mnt/space/{work,snapshot}

• sudo nano /etc/refractasnapshot.conf
  # find variables: snapshot_dir and work_dir and edit them to contain /mnt/space/snapshot and /mnt/space/work

• get your new kernel version:

  uname -r # this shows the correct format that you need, but the old kernel version number
  ls -l /vmlinuz # this is the new version; take the matching bits from the end of this line
  sudo nano /bin/refractasnapshot
  # at approx line #438, find line starting with "mkinitramfs"; to the end of that line, add a space and your kernel version, e.g. 6.12.86+deb13-amd64

  the above edit is a hack, and could be better if scripted

• sudo refractasnapshot
  # choose: 1
  # type: q
  # hit: <Enter>

  when done, find your newly upgraded .ISO in the /mnt/space/snapshot/ directory (under snapshot/ on your storage-media)

I've just finished doing the above and am running on the upgraded desktop-live .ISO to make this post.
(also with this upgrade, now safe from Copy Fail CVE-2026-31431)

thanks to fsmithred for prompt advice and encouragement

OK, I think the message is becoming a bit more clear.

If the .ISO is an egg:

The idea I had was to make an egg from an egg.
One could set up a genetics lab to do it.

... or just let the egg hatch into a chicken, and then let the chicken lay it's own egg.

(there must be an ethnic proverb out there that covers this)

OK, so if I get what you're saying, fsmithred, the proven method would be:

• boot computer(or virtual machine) on the live .ISO

• insert (if not already present) some empty storage-media, or some storage-media that can be overwritten

• install the live-system onto that storage-media

• then boot that newly-installed system off of the storage-media

• now upgrade that booted system (and/or modify it to your liking)

• lastly, run refractasnapshot to make a new, upgraded live .ISO of your installed system

Afterward, keep the operating system that's on the storage-media for future-use (preferably clean, and otherwise unused).  Upgrade or modify it periodically, then run refractasnapshot to create another new, upgraded live .ISO

Looking at it that way, I see an extra step involved (creation of an installed-system on storage-media), versus what I was attempting to do.

However, if the user upgrades their .ISO more than one time between .ISO releases, then network-bandwidth will be saved between user and devuan repositories (by not re-downloading the same updates each time they update their .ISO).

Do you agree or have corrections or anything do add?