Not using the old sand will take all of the beneficial bacteria that lives in the sand and eats the poop and algae and will help with cycling the ammonia and nitrites out of the new system. That said I think there are also gasses that build up in the sand that you don't want released into the new system (which is what will happen when you disturb and move the sand) or at least that is what I have been lead to believe...thus I am asking the community for their opinion and knowledge.