While not really an appropriate question for a "Vintage Computing" forum, I'll give it a shot:
It depends on the system. If it's a fairly conventional laptop or desktop, it will probably 'downgrade' just fine. Look on the manufacturer's website first to see if they have Windows 7 drivers, though.
If it's a tablet, "2 in one", "convertible" or anything similar - almost certainly not. Those type systems tend to have a tightly-integrated copy of Windows with custom drivers for the tablet portions. Almost none of those can run Windows 7 acceptably (you'd be missing many drivers, sometimes for vital parts of hardware like the display!)
