While you'll need a genuine copy of Windows 7 or 8 to get a free upgrade to a licensed copy of Windows 10 you will be able to keep running the OS without buying it - if you're willing to keep testing it.
People testing early builds of Windows 10 can continue running the OS after its release in July but not every tester will be eligible for a free upgrade to a licensed copy.
Microsoft issued the clarification after widespread confusion over whether people testing Windows 10 as part of the Windows Insider Program (WIP) would get a free upgrade.
Gabe Aul, Microsoft's general manager of the operating systems data and fundamentals team, has confirmed that testing the OS won't automatically make you eligible to be upgraded to a licensed copy.
Instead, WIP members will continue to get early builds of Windows 10 after the OS is released on 29 July, so they can test them ahead of final builds being pushed to people running licensed copies of Windows 10.
So, while in effect people testing Windows 10 will be able to continue running the OS without paying for it, they will be running pre-release, and by implication less stable, builds. However, if those testers are also running a genuine copy of Windows 7 or 8 they will be eligible for the free upgrade to Windows 10, which Microsoft announced earlier this year.
"As part of the program we'll upgrade Insiders to what is for all intents and purposes the same build as what other customers will get on 7/29, but that will be just another build for Insiders, and those who stay in the program will simply get the next build after as well," said Aul.
To continue receiving new builds of Windows 10 after the release on July 29th WIP members will need to be running the Windows 10 Insider Preview (Home and Pro editions) and have a registered Microsoft Account connected to their PC.
Aul doesn't indicate that Microsoft has any plans to halt the testing program in the short term, suggesting testers can continue running Windows 10 for the foreseeable future. Each pre-release build will expire after a set period but before that happens testers will be moved to a new build.
Microsoft will also provide ISOs for these early builds to allow them to be recovered from any significant problems.
WIP members who choose to opt out of the testing program after receiving the release build of Windows 10 on July 29th will have a few options. If they upgraded to an early build of Windows 10 from a genuine copy of Windows 7 or 8 they qualify for a free upgrade and can continue running an activated copy of Windows 10. If, however, they installed Windows 10 from a different OS, for example Windows XP or Vista, they will be required to switch back to their previous OS or buy a Windows 10 license. If they don't do that then the build will eventually expire and they will no longer be able to use Windows 10.
Most owners of Windows 7, 8 and 8.1 operating systems will see a Windows icon in their taskbar that will allow them to "reserve" their free upgrade to Windows 10. The 3GB file can be downloaded from 29th July. The free upgrade will be available until July next year and those choosing to switch to Windows 10 can cancel their reservations at "at any time", according to Microsoft.
To upgrade using Windows Update users must be running Windows 7 (Service Pack 1) or Windows 8 (Windows 8.1 Update). Other Windows 7 and Windows 8 users will still be able to upgrade to Windows 10 by downloading the ISO image from Microsoft.
While Windows 7 Enterprise, Windows 8/8.1 Enterprise and Windows RT/RT 8.1 releases can't be upgraded in this way, Microsoft has said that "Active Software Assurance customers in volume licensing" will be able to "upgrade to Windows 10 enterprise offerings outside of this offer". Microsoft are yet to clarify when users of Enterprise versions of Windows will be able to move to Windows 10 Enterprise - although there are rumours it will be in September or October this year.