There's no need to create a new partition. You can upgrade Windows 7 to Windows 8 in place the same way as you would on a Windows machine. Just boot to Windows 7, insert the Windows 8 DVD and it should automatically start. From there, follow directions. Once Windows 8 is installed, you'll need to upgrade drivers by running the Boot Camp update from Control Panel.
One other thing... before upgrading make a backup of your Windows 7 installation. I recommend using WinClone. Not free but it provides the best backup protection.
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