Forums will be making significant UX changes to address key usability improvements surrounding search, discoverability and navigation.
To learn more about these changes please visit the announcement which can be found
1 - Reinstall WIndows so your COA matches your Product Key (install using your COA as the Product Key) - This is the preferred method by Microsoft, as it will be a fresh copy, void of potential hacked binary files and security loops that may have been created to bypass WGA. We know this is usually not the user preference, but it is the safest method.
2 - WGA is underway to create a tool that will allow you to switch your current invalid/non-genuine Product Key being used to your COA sticker *ONLY*. There's no slated timeline, but it will be soon. This will allow you to change product keys being used without a reinstall.