If I recall there actually is a script command to do this. I'll have to do some digging to find it.
*setnpcdisplay("<npc name>", "<display name>", <class id>, <size>)
*setnpcdisplay("<npc name>", "<display name>", <class id>)
*setnpcdisplay("<npc name>", "<display name>")
*setnpcdisplay("<npc name>", <class id>)
Changes the display name and/or display class of the target NPC.
Returns 0 is successful, 1 if the NPC does not exist.
Size is 0 = normal 1 = small 2 = big.
I believe it's the class ID you want to change.