One important note is that unlike the current Windows 8 installation media, the Windows 8.1 discs and digital distribution versions will no longer be upgrade-only, they will be "full version software" that do not require a previous version of Windows in order to be installed. The pricing of the OEM versions is unknown.
One shift to note in Windows 8.1 is that we will be offering “full version software” at retail and online for download that does not require a previous version of Windows in order to be installed. The copy of Windows 8 that is currently available for sale at retail and online is an “upgrade version.” This shift allows more flexibility for customers in specific technical scenarios and is in response to feedback we’ve received. It will be easier for those consumers who want to build PCs from scratch, run Windows 8.1 in Virtual Machine (VM) environments, or run Windows 8.1 on a second hard drive partition. For non-Windows 8 devices, here is how upgrading to Windows 8.1 using the retail DVD and download software will work:
Windows 7: Consumers can upgrade a Windows 7 PC which will bring along all their files, but will require them to reinstall desktop apps including Microsoft Office. Windows XP & Windows Vista: Windows 8.1 is not designed for installation on devices running Windows XP or Windows Vista. Although not designed or recommended for devices running Windows XP or Windows Vista, consumers still wanting to upgrade from Windows XP or Windows Vista should buy the retail DVD instead of using the download and boot from the DVD to do a clean install of Windows 8.1. Note: files, settings and programs will not transfer – Consumers will need to back up their files and settings, perform clean installation, and then reinstall their files, settings and programs.
If you buy a device later this year that comes with the Windows 8.1 edition, you will be able to purchase the Windows 8.1 Pro Pack for $99.99 ERP (U.S.), which unlocks all of the great features unique to Pro, as well as Windows Media Center. If you are on Windows 8.1 Pro, you can buy Windows Media Center for $9.99 (U.S.).
Source: Windows Blog