The only way to know if you like Windows 10 or not, of course, is to upgrade your system and give it a try. (Some 14 million people did just that in the first 24 hours of availability, Microsoft reported.) However, if you want to go back to your old OS, it appears that you're on a timer: Microsoft confirmed to Tom's Hardware that users will have only one month to decide if they want to continue with Windows 10 or not.
A Month To Decide On Windows 10?
A moderator on Tom's Hardware (darkbreeze) first discovered this issue, and when we reached out to Microsoft for more information, the answers we received raised some troubling potential issues.
Our initial thought was that this "one month" business simply meant you couldn't revert directly back to an old OS from Windows 10, but what about rolling back by simply performing a clean installation of your old OS? When we asked about that specifically, Microsoft dodged the question.
At this point, then, we aren't certain if you can return to your old OS by using a clean install, because it is possible that Microsoft will invalidate old activation codes after that first month. This would seem in some way to align with Microsoft's upgrade deal, as you can only get your free Windows 10 by upgrading from your old OS; you can't use an old activation code.
The Hardware Issue
If you want a clean install of Windows 10, you must first upgrade from your old OS to Windows 10, and then re-install Windows 10 again (from installation media). Even though it formats your hard drive during installation, Windows 10 somehow remembers that you previously upgraded from an older OS, and it will self-activate after installation.
It isn't clear how Windows 10 does this, but it's likely because it saves the activation code somewhere else inside of your hardware. Some sort of online account-based activation would be far more conventional, but there is reason to suspect this other method is being used.
Microsoft informed us that a "meaningful" change to the hardware may require you to contact customer support in order to activate the system. This means that if you upgrade your system with a new motherboard or CPU, you may not be able to install Windows 10 without having to go through Microsoft's customer support, and even then, you may not be able to activate the system.
Microsoft has stated that even with customer support, some hardware changes will invalidate your free copy of Windows 10 and require you to purchase one. (This limitation does not exist in retail copies of Windows 10; those can be re-used by the user regardless of any hardware changes.)
This changes the possibility of deactivating old activation codes from an irritating problem for some users to a major problem for countless users. Tech enthusiasts stand to be hit hardest by this, as they might change the hardware inside of their systems multiple times a year. Now, these users will have to choose between upgrading a PC part or keeping their free copy of Windows 10.
The problem gets worse for users who have a part go bad and need a replacement, as they don't have a choice; they need to fix the part or they can't use their PC, and the cost of that repair, then, would also have to include whatever Microsoft is charging for a new copy of Windows 10 at the time.
You could simply go back to your old OS at that point to save a buck, but -- again, if Microsoft invalidates your old activation code, you may be stuck having to buy a new copy of Windows 10 anyway.
This policy from Microsoft was likely developed to avoid users installing the older OS on a separate system after upgrading to Windows 10, which is a perfectly acceptable thing for a company to do. After all, Microsoft still needs to make money, and if it just lets anyone use those old licenses inappropriately, then it would lose a lot of money from potential OS sales.
From the perspective of the consumer, however, these issues seem to jibe with Mozilla's assertion that Microsoft has reduced user choice in Windows 10, in this case by giving them very limited ability to downgrade the OS, re-install the OS or change hardware. The initial upgrade process itself is needlessly long and complicated, too.