I know this might seem counterintuitive but don’t change that desktop. You are willing to take the chance and you might be excited for it, your family might accept it but they’re not excited about it. This means they will be a lot less tolerant of issues, and there will be issues. LibreOffice (or whichever you use) is not 100% compatible, there are corner cases, there are things that don’t work, and depending on what your father does with those documents it might be impossible for him to do it, if it’s work he might even get reprimanded for it. If you go down that road try the alternatives first, most Linux programs are available on Windows, so you can install Firefox, OnlyOffice, and any other they might need and see if you all adapt well to it in the security that you can simply drop the test and go back to what you were using before. Not having a backup is dangerous in and of itself, but not having a backup when purposefully formatting the drive is shooting yourself in the foot deliberately. I would recommend you try to get your own system, an old laptop that needs to be plugged to work will do, I’m sure you can find one very cheap somewhere.