Halo 2 is more than likely using the majority of the original code base with updated textures and models. LA will have been made from the ground up.It's tile-for-tile the same game, is it not?
It's more like the Halo 2 remaster except you can't switch back to the old graphics.
Halo 2 is like colouring in a picture, LA is like freehand drawing from reference and then colouring it in.