At the WinHEC conference in China, Microsoft has announced several new details about the release of Windows 10, including that all pirated copies of Windows can be upgraded for free. If you are wanting to upgrade to Windows 10, take a look at the matrix below, as it outlines your options for installing the OS.
To no surprise, Microsoft will offer upgrades of Windows 10 via ISO and Windows Update. Generally speaking, if you are running the latest version of Windows 7 or Windows 8, you will have the option to use an ISO or streamline the update process by using Windows Update. For all other iterations of Windows 8 and 7, your only options will be to use an ISO.
For those of you with Windows Phones, anyone running Windows 8.1 will be able to upgrade to Windows 10, per the matrix, but if you are still running Windows Phone 8, there is no upgrade path. This means that you will need to upgrade your phone to Windows Phone 8.1 if you wish to install Windows 10 for phones.
Windows RT appears to be dead based on this chart, Microsoft previously said there would some sort of an update pushed out for those devices but based on this chart, the platform will not be able to run any form of Windows 10.
If you are running Vista or even XP, you will need to do a clean install with an ISO.
The upgrade channels for current Windows users are straight forward, and considering that the OS is free to upgrade, we would expect the adoption of this OS to vastly dwarf that of Windows 8 when it is released later this year.