You can do that by changing the data in the database, creature_template, modelid1/2/3.
Otherwise you should be able to do it ingame by typing .npc set displayid XX
For that, you need to find the displayid that you would like to change it to, and the entry of the guards (which you can find by selecting the npc and type .npc info)
So I didn't have any trouble changing the appearance in the database, but my efforts to change the name of the model seems to be meeting with failure. I changed the name in the Creature Template, but I haven't found any other place to rename. Is there something I'm doing wrong?