You can do a fresh install with an upgrade key. Thats no problem. Just make sure you boot from the disk instead of starting the install from Windows. So long as the the install dectects you had windows installed on that system it will count as an upgrade. You can even format the drive from the installer if needed. You can use the license later if you want thats up to you. Not sure about how the licensing thing works but I wont tell if you dont tell.
Also I recommend that if your system came pre-installed with 7 that you create the recovery media so you can get back 7 if you do not like 8 for some reason. You could always install 8 to a VHD and run the 2 OSes on the same drive without having to repartition anything just to see if you like it or not.