It shouldn’t be any surprise that if you run the Windows Media Creation tool, use it to “Upgrade now,” and in the dialog marked “Choose what to keep,” specify Nothing, you won’t be able to roll back to your original programs or files. This is a common technique for performing a clean install of Windows 10 -- highly recommended to make sure Win10 is more stable. Unfortunately, it also removes your ability to go back to Win7 or 8.1.
In the same vein, if you upgrade to Windows 10, use either the Media Creation Tool or the Windows 10 “Reset this PC” function (Start > Settings > Update & security > Recovery), then tell Windows that you want to “Remove everything / Removes all of your personal files, apps and settings,” the key folders will be removed, and you can’t revert to your old version of Windows.
I’ve seen a lot of advice for recovering the three key hidden folders, should they be deleted. Unfortunately, I haven’t witnessed any approach that works consistently.
That thing about the 30-day clock
After 30 days, you're up the ol' creek without a paddle. If you want to go back to Win7 or 8.1, you have to re-install it from scratch, and you're responsible for moving your apps and data.
If you made a system backup before you upgraded to Win10, you can, of course, go back to that backup. Usual system backup rules: What you get is an exact copy of what you had at the point you made the backup.
If you're coming close to your 30 days, and are the cautious type, you should consider rolling back (taking into account the disappearance of files in unusual places), then rolling forward again. That resets the clock, so you get an additional 30 days to see if you like the Win10 experience.
It's not clear how Microsoft sets the 30 day clock. You'd think it would be a Scheduled Task, but I looked high and low and couldn't find it. (I was anticipating a hack where you could re-schedule the task manually.) But what is clear is that once the files necessary to roll back are wiped out, you're SOL.
What to do if the wheels fall off
In my experience, the rollback to Windows 7 and 8.1 works remarkably well, given the caveats mentioned previously. I have heard of problems, though, ranging from icons that don’t display properly on the recovered desktop, to missing data, to programs/drivers that aren’t working correctly, even though they used to work fine.
If you can’t get Windows to roll back and absolutely detest Windows 10, you’re up against a very tough choice. The only option I’ve found that works reliably is to re-install your original version of Windows from scratch. On some machines, the old recovery partition still exists, and you can bring back your old version of Windows by going through the standard recovery partition technique (which varies from manufacturer to manufacturer), commonly called a “Factory restore.” More frequently, you get to start all over with a fresh install of Windows 7 or 8.1.
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