You’ll be able to purchase Windows 8.1 full version from local store or download full version from Windows official website with $119.99 for Windows 8.1 and $199.99 for Windows 8.1 Pro. If you purchase new computer in future and if it already comes with Windows 8.1 installed, you’ll be able to upgrade to Windows 8.1 Pro for $99.99. If your computer contains Windows 8.1 Pro, you’ll be able to purchase Windows Media Center pack for $9.99.
FREE if you upgrade from Windows 8 to Windows 8.1.
Your personal documents and files will be ratain but you’ll need to install all 3rd party software again if you upgrade from Windows 7 to Windows 8.1
Not possible to upgrade from Windows XP or Vista to Windows 8.1. You’ll need to perform clean installation of Windows 8.1.