I would let the tank finish cycling and then add the new sand. I'm guessing you intend to do a DSB
? If so, after it matures, it will help you control nitrates in the tank. It'll also make the step up to a reef tank much easier if you decide to do that later. And yes, the "dead" sand will become live sand over time. If you have friends who have reef tanks, try to talk them out of a small cup of their sand after your tank cycles. That will increase the diversity of critters you'll have in your sand.