I manage to make a default replacement of the private school uniforms. I also had to make a new default mesh, but the thing is, the original mesh is used by more than one textures, so when i change the default mesh, it effects about 7 other formal wear textures for the males, and the other 3 private school everyday wear for the girls. I was wondering is there a possibility that i can change what mesh that texture is link to, and if i do that will it still be a default replacement? (( I hope you understand what i am saying?)

Someone more knowledgeable than I (and there are plenty!) can correct me on this, but I believe that there is a BHAV function that sets the Private School Outfit.
If you make your outfits stand alone, you may be able to change that function to call up the GUID for your new mesh/texture instead- thereby not overriding anything.