InfinityRider,
The Windows 8 CD key is a single use key, unless otherwise stated on the CD or packaging. It may be worth the time to call Microsoft and see whether or not they will let you reuse the key, before you buy another copy of Windows.
If you do not want to buy a new copy of Windows, then you can perform a Disk Clone. There are several reputatable, free cloning software available. You will need a way to connect both HDD to your computer, though.
Yimman