davidfor and frostschutz.
Your comments made me understand I was lacking knowledge about terminology and correct Kobo firmware management. Although I should by now understand it correctly, I was assuming firmware as simply replaced with drive copy/paste...
As far as I now understand, no matter what firmware version is currently installed, copying my full drive backup to Kobo will restore it to the previous state.
It's clear now to me and I already have it restored as I wanted.
Thank you very much for your time!