im going to buy Avid disk breakes mechanicals in a whelset package there the 160mm disks...later on when the rotor wears out can i swap to 203 mm?? or do i need to make modifications

Your thinking too far into the future.....the rotors are going to last you a long while.......and yes, you will need to get an 8" adapter for a 203mm rotor.....just get another 6" after yours wears out, you dont need an 8.