If you got the game installed by steam, you cannot simply change the cd key in the registry. By starting the game with steam, steam always changes it back to the steam key.
That means, if you uninstall warband on steam and use a normal .exe (download at taleworlds page), you can change the cd key in the registry any time.
If you install warband with a steam account, only the steam cd key on that account for the game will work.
So you must either install the game with the steam account logged in when you played your character with that steam account before or you take a non-steam .exe .