Free upgrade to Windows-10 is possible only if the licensed and officially registered version of Windows-7.8, or 8.1 was used before it.
On Windows 10, it will be possible to update for free from Win 7/8 / 8.1, both with a pirated and licensed copy. But this will work for a while. Then just buy. You will not have to pay for already updated copies of Windows. This is most likely done to re-capture the market and make Windows 10 more popular. It was the same in Windows xp, it seemed like a license and only a sale, but there were a lot of pirated mines and no one really followed this, and therefore almost all computers were running Windows xp.
They promise that in a year it will become paid. Now it seems in the license agreement, there is already this point. That is, you can upgrade from the pirated version, but after a year you will have to pay.
As far as I know, upgrading to Windows 10 will be free only for users of licensed versions of Windows 7/8 / 8.1. And as we know, most people use "pirated copies", so ... Wait for a pirated copy of a dozen.
I don’t think, because for Microsoft the free version of the OP will benefit, as well as users. In exchange for a free operating system, users will have to report their experience of using Windows 10. This will need to be done using a special application, the communication with which, according to the developers, will be simple and easy.