What Jackel said. Be very careful with .utc editing. You can use tk102's findrefs to verify if the file is unique: 
http://www.starwarsknights.com/tools) 
 If the .utc is not unique, then your best bet is scripts. 
 In KotOR 1 you can apply a disguise: 
 void main() 
 {
 //check nwscript.nss for the possible disguise effects (DISGUISE_TYPE_*)
 ApplyEffectToObject(DURATION_TYPE_PERMANENT,DISGUI SE_TYPE_*, GetObjectByTag("my_npc_tag")); 
 }  
 In Kotor 2, you can use the ChangeObjectAppearance function:  
 void main()
 {
 //Replace int Appearance by the appearance.2da row number.
 ChangeObjectAppearance(GetObjectByTag("my_npc_tag"), int nAppearance );
 } 
 The duplicate head appearance function can also be useful depending on what you want to do (just avoid putting Vrook's head on Mission's body!): 
 DuplicateHeadAppearance(object oidCreatureToChange, object oidCreatureToMatch); 
 However, I don't know if this function is available in KotOR 1. You can verify this by checking nwscript.nss.