Unable to upgrade Windows 10 edition from Home to Pro

I had exactly this problem.

I solved it buy obtaining a new Windows 10 Home key from msdn and then changing the product key (also via the settings-> Activation window). After doing this I then repeating the attempt to update to Pro (with legit key) via the store.

My guess is that whatever I had as a key was not eligable for the upgrade combination i was trying via the store.