If your not having nitrate or phosphate issues, I would use as much old water as you can. Just less new water for you to have to make and waste money on. Like ^^^ better to put them into similar if not matching parameters when you transfer everyone over. Either way really there is no best answer to your question. Better to err on the more side. Most of your bacteria will come over on your live rock from your current tank. I always added new sand when I upgraded tanks and tried not to disturb the sand bed.