You can’t change the CD Key of Windows 8 Enterprise by going to the System screen. To change Enterprise to Pro, you need the Windows 8 retail CD along with your windows 8 Pro CD KEY. Popin the CD in your computer and reinstall windows however, when prompted what you want to do with your apps, files, and settings make sure you select the KEEP Settings, Apps, and Files!! It should be the default selected checkbox but make sure just in case.
Hope this helps other folks out there.