I feel for you. It must feel like being robbed of your memories and achievement. Although I have little experience with taxidermists, the ones I've seen seem to be chaotic and its a miracle that anyone gets their right trophy. Of course most people don't mark their stuff and will never know whether they got the right one or not. All to easy for the taxidermist to fob them off.
If you lawyer up it'll cost a lot of money. Only you can judge whether its worth it. First off though I'd be talking to the taxidermist and advise him that you've scanned the skull and its definitely not the one you marked. It may be an honest mistake on his part that he can rectify if the correct skull hasn't already gone out to someone else. If he is still unwilling to help then you need to decide what to do next. Either write it off to experience, take legal action, or expose his conduct on all the local hunting forums.