If you purchased a boxed copy in a store or from a different web site than this one, then your copy was published by Kalypso, and they have a different registration system than us. You can tell you have a Kalypso edition because the serial codes are 28 characters long and start with 'HEG'.
From my understanding, the Kalypso launcher should be offering to update your copy of the game, but it doesn't seem to be doing that, so I'll ping Kalypso to see what's up.
In the meantime, the other option is to install the Kalypso version on your disc, and then install our demo over top of the Kalypso edition. Don't worry, you'll still have a full version of the game; this will just update the exe and data files to the latest version.
my attempt at an update resulted in not being able to play the game at all. even the beta mod is asking for a product key (which I already entered the first time around and which is no longer recognised after the update).
posted about the problem here: http://www.longbowgames.com/forums/topic/?id=3292&start=0#post27632