Ok, so I have a retextured version of the guard cuirass, but I only want it to replace the texture on the arms, and not the main upperbody. I tried, but it also changed the texture of the upper, with the arms. So I tried copying the retextured arms, returning the texture to normal, paste the retextured arms in again, and remove the normal-arms. Whenever this is loaded in the CS, the arms are invisible. I don't know how to fix this, either by making the arms show or quit forcing the same texture for both parts. Any help?
ps: The Cuirass is the Skingrad one, but I tried and also failed at doing the same thing to the Bruma.

Forgive me if I misunderstand what you are doing but why not just retexture the arm parts of 'textures\armor\townguardsk\m\Cuirass.dds'? You can easily do this by adding your new texture in a layer below the original and erasing the arm parts in the original texture. NifSkope shows what parts of the texture are used for the different parts of the mesh and you can even export this data and use it as a reference layer in your graphics program.

If I understand correctly your problem is that they have combined properties on the seperate Ninodes in the NIF so that changing the texture path on one NiTrishape also changes the other Nitrishapes that were using that texture -- If this is the case then to fix it you need to open the cuirass Nif and on the top menus go to SPells - Optimize - Split Properties (this seperates all of the Nitrishapes that use the same properties so that they can be individually changed without effecting the others using the same properties (ie. texture path) ) -- make your changes and then run the Spells - Optimize - Combine properties (this will relink the ones remaining that use the same properties to optimize performance of the NIF ( so for example it will only load the texture used once for the entire nif instead once for each Nitrishape to save memory usage)

The copy\paste should also work but most likely you did not make a normal map texture of your new texture ( _n.dds file) without this the game does not know how light should interact with the texture and therefore no light reflects so the texture is invisible ( that or you did not paste the arms in properly so they are a seperate nitrishape not attached to the rest of the model so is not displayed in game)