Here is an issue I see happening with windows 10 upgrade using windows update. You get it all installed. Everything is running fine, then the following year your hard drive crashes or your windows 10 operating quits working. Now you can't use an iso or dvd or flash drive to reinstall windows 10. If you put your old system back on, you won't be able to get the free upgrade anymore because it been more than a year. Now you half to go back to your old system or buy a windows 10 dvd.