If you haven't opened your forums yet and no one has used the smiley codes then yes, rename them.
However, you can have multiple "codes" for the same smiley so instead you could simply add the German code
above the English one (to make it the default for that smiley) whilst still allowing the existing English ones to work because of the English code underneath.
So when you edit the cup of coffee smiley, you simply make the codes:
... and when someone inserts the coffee smiley your board will use :kaffee: as the new code, but still allow the old ones to work too.
Win win