With questions like this one what I usually do is simulate it in a vm so I can see what happens before I reply to make sure my answer will work. I installed pureos in a vm, attempt to convert to debian with the constraints given by the opp. After several start-overs/re-installs in a vm, I came up with my answer.
The issues that I addressed when I started to work on this:
1. Getting the iso to boot and start the installer
2. Removal of the current distro so as not to cause problems with the new os and retention of home if desired.
3. The ability to reboot back into the iso if something goes wrong.
4. attempt to consider all scenarios in which this can go wrong.
5. complication of instructions.
I installed puros in a vm, the first thing I tried was your post 16 to boot the iso.
The iso booted, asked for langauge, keyboard, hangs at: "no media installation detected Do you want to install drivers?"
Went here
https://www.debian.org/releases/stable/i386/apa followed the link for hard disk.
The only remnants of puros are the puros kernels in /boot and some configuration files in /home/. I did not suggest to delete the puros kernels because if the wrong ones are deleted will result in a broken system.
