If you upgraded your Windows XP, Vista, or Windows 7 system to the Windows 8 Release Preview, you’ll be happy to know that Microsoft is expanding their offer and it will allow you to upgrade to the final version of Windows 8 Pro for $39.99 as well. However, you need to provide a previous genuine Windows license in order to qualify for the upgrade.
Microsoft said that if you installed Windows 8 Release Preview on top of your previous installation (Windows 7, Vista or XP), or you did a clean install because of the previous version of Windows, you still qualify for the upgrade offer. Now, for example, if you installed RP on a Mac OS X, you will have to pay for a full version of Windows.
Remember that the cheaper upgrade path will be available on the web as a downloadable media at the time Windows 8 releases in the coming months. An exact date for the final release is yet to be announced, but it could be as early as mid-October, according to online rumors.
Source The Verge