There are only 2 edition for Windows 8 you can normally purchase. However, there are several types of packages you can choose from. Below are packages you can choose, and some have different package designs (only design differ, no difference inside).
Windows 8 Pro Upgrade
Package you can purchase at store. Used to upgrade from Windows XP, Vista, or 7 to Windows 8 Pro. Includes media for both 32bit and 64bit version. Cannot be installed on empty system (no clean install). 5 different package designs.
Windows 8 Pro Upgrade Download Ver
Windows 8 Pro upgrade you can purchase and download from Microsoft homepage. Upon upgrade, you need to prepare USB or DVD media.
Cheaper than package, but if you are using XP or other 32bit version and want to upgrade to 64bit, steps will be complicated.
Windows 8 Pro Pack
Package you can purchase at store. Used to upgrade from Windows 8 to Windows 8 Pro. Include both 32bit and 64bit media. Unless you purchased Windows 8 dsp or PC with Windows 8 preinstalled, you won’t need it.
Windows 8 DSP
Windows 8 which can be clean installed or upgrade from older Windows. Note that only one of 32bit or 64bit media is included, and you won’t get Microsoft support.
Windows 8 Pro DSP
Windows 8 Pro which can be clean installed or upgrade from older Windows. Note that only one of 32bit or 64bit media is included, and you won’t get Microsoft support.
I believe upgrading to Windows 8 from already using Windows is most of the case. So, you should be buying Windows 8 Upgrade package. If you are building your own PC from parts, you will be purchasing dsp version.
Difference between pro and non-pro
Pro includes support for remote desktop, domains, and BitLocker (HDD encryption). For difference between Enterprise and RT, refer to Wiki for detail.
Upgradable or not
If you are using Windows XP, Vista, or 7 and willing to upgrade to 8, there are many restrictions. Some software might be needed to uninstalled before upgrading, and moreover, you can only keep your software if you are currently using Windows 7. You can’t take your software with you from XP or Vista.
Well, try Upgrade Assistant from Microsoft and this will make you a report whether if your environment can be upgraded to Windows 8 or not.
One thing to note; upgrade assistant will warn you incompatible software, but won’t warn you incompatible drivers. Therefore, you might have some peripherals not working after upgrading.
Upgrade between 32bit/64bit
No matter what OS you are using, you can’t upgrade between 32bit to 64bit or vice versa. You can keep your files on HDD, but OS will be clean installed. If you are switching 32/64bit, I recommend backing up all your files to external storage, and clean install from formatting HDD.
Upgrading from Windows XP
If you are using Windows XP, even upgrading to Windows 8 32bit version gives you option to only keeping personal files. All software installation and settings must be redone. If you PC supports 64bit, I recommend clean installing Windows 8 64bit version.
Upgrading from Windows Vista
If you are using Windows Vista, you can keep Windows setting and personal files, but all software must be re-installed.
Upgrading from Windows 7
If your Windows 7 is Professional or Ultimate, you can’t upgrade to non-Pro Windows 8. You can upgrade to non-Pro if your Windows 7 is Starter, Home Basic, or Home Premium. All edition can be upgraded to Windows 8 Pro, and all software, settings, and personal files can be kept (there may be some that might not work in 8, though).
Check with Upgrade Assistance before upgrading so you can know what software might be needed to uninstalled before upgrading to Windows 8.
One of recommendation is to keep your current OS for a while, install Windows 8 to different disk partition making dual-booting, and eventually moving to new OS, and delete old OS when no longer needed.
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