Windows 10 Insiders who installed Windows 10 Technical Preview or Windows 10 Insider Preview on their computer will have direct and free upgrade path to final RTM (Release to Manufacturing) version of the OS when Windows 10 is officially released.

It means that Windows Insiders can upgrade to Windows 10 RTM build without the need of performing clean install or restoring back to original Windows 7 or Windows 8.1. As explained by Gabriel Aul on Twitter in early May, the public face of Windows Insider program, officially, you still need a valid license for Windows 7, Windows 8 or Windows 8.1 to upgrade from preview build to genuine Windows 10 RTM to avoid been detected as pirate.

In addition, Gabriel Aul also explained that Windows 10 final build will automatically available to Windows Insiders when Windows 10 is released on July 29th. Windows 10 is not required to reserve the free upgrade. Early June, Microsoft activates the free Windows 10 upgrade reserve program to allow Windows 7 and Windows 8.x users to opt in to automatic Windows 10 downloads when released, though those not reserved will still able to upgrade to Windows 10 for free within the first year of release.

Best of all, no product key of Windows 7 or Windows 8.x is required to upgrade.

In addition, system running Windows 10 Insider Preview and connected to Microsoft Account (MSA) is eligible for free Windows 10 RTM license.

Windows 10 - Important Information