Upgrading to Windows 10 from Windows 7 and 8 will be free, but buying boxed copies of the operating system—for installing onto newly built PCs, for upgrading from Windows Vista or Windows XP, or for running inside virtual machines—will not be free. It will cost up to $199, depending on whether you buy the Home edition or the somewhat more capable Pro edition.

Or alternatively, it will cost nothing at all, according to a process outlined by Microsoft in a new blog post. If you're running the Insider Preview release of Windows 10, currently build 10130, you'll be able to upgrade to the final build (and all subsequent releases) on July 29. This version will be genuine, which is to say "not pirate."

The Insider program for Windows 10 is changing in the run up to the first stable release. Currently, preview builds are all automatically opted in to the Insider Program (obviously; that's the only way to even get them). When Windows 10 is launched, that changes: at that point, the Insider Program becomes opt-in. To opt in, users will have to have a Microsoft account that's registered with the Insider Program, and they'll have to use that Microsoft account with their PC. This change will take effect in the next public build: without a suitably registered Microsoft account, that build will be upgradable to the final release on July 29, and from there on out be restricted to regular, stable builds.

This does, of course, impose some limitations on the "free" install process. In particular, it's unlikely that build 10130 or the other pre-release will be installable and activatable forever, so the window for getting onto the train is limited.

We also wouldn't be tremendously surprised if there were some kind of volte-face that will in some way attempt to distinguish between Windows that passes the genuine Windows checks and Windows that's truly genuine. In broad strokes, Windows licensing isn't all that complicated, but there's a vast number of awkward corner cases, especially when it comes to upgrading, and nebulous concepts such as the supported lifetime of a device. Combine this with consistently unclear statements about "re-engaging" with software pirates, and throw a time-limited "free upgrade" promotion into the mix, and the result is that many Windows users aren't clear whether they'll get Windows 10 or not, or if they do, how long they'll receive updates for the OS. About the only thing we know for certain is that free upgraders will still be able to perform clean installs.