The '03 Belt has smaller teeth and is 12mm wide versus the larger teeth on the '04+ belt and 14mm wide.
This also requires new pulleys as the teeth are a different size
Bike on the right '04+ pulley setup on my Ss swing arm swap, bike on the left '03 pulley setup. You can sort of see how the '04+ pulley os larger, there isnt any power difference as the front pulley is different as well but just a side by side.