There is a Pro key and a Core key? The one you use to install has to match the 8 key your going to activate with.
You can those key.
NoteNote that although you may previously purchase a legit Windows 8 product key, the product key is unlikely able to be used to install or upgrade to Windows 8.1. Hence you will need the following product key to initiate the install or upgrade:
Windows 8 (core): 334NH-RXG76-64THK-C7CKG-D3VPT
Windows 8 PRO: XHQ8N-C3MCJ-RQXB6-WCHYG-C9WKB
The keys above will let you install 8.1, but that's all it does. After installation is complete, you will still have to activate with your own Windows 8 key.