Rob,
Lets look at this this way, the individual armors are not unique they are a texture variant applied over a common Body Model also used by other armors in the game. To replace one will affect all those that use that model.
The Republic Mod Armor is Texture Variant 1 of Body Model Class 5 (Light Armor)... to replace this like you want you would also automatically replace all other Body Model Class 5 armors in the game (No Matter the texture variant they use)... all of them not just the Republic Mod Armor, all the others too, including the Bondan Alloy Heavy Suit, The Zabrak Armors, Eradu Prototype, Bargawin Shadow Armor, Light Exoskeliton, even the rare Ulic Quel-Droma armor which also is class 5 will all become the Republic Uniform... hence why we don't generally go messing/replacing the armor/robe body models in the game, we just skin new texure variants on the normal body models as 'new' armors, like Achilles did in the link I gave you above.
Once again I'll repeat... You cannot do what you want without changing all the armors that use that body model in the game! To see which exact ones you would replace, look here (
http://www.starwarsknights.com/itempages1/armor.php) any of these with a 5000 series number in the cheat code string next to their names would be affected in game by your requested Class 5 Body Model model change...
That's why most mods add the NPC body models in the clothing slot, because if you overwrite one of the armors and robe slots you are messing with too much to handle in the game.
But if this doesn't matter to you, you can indeed replace an armor with the Republic Uniform Body, but be warned you will have issues in the game with the Republic Model appearing on all NPC's who wear that class of armor.
Though if you really want to do this, and the 'bugs' won't bother you, and you want instructions, we will be more than happy to post them for you. :)
I hope I was clearer this time! :D