Starting again with new sand is certainly not a bad idea, but the existing sand (if a 1" bed) will help seed the new tank. You can't immediately add everything back in anyway. If most of your "new" sand is also live sand, then use that. If you get ammonia, which you can get with the old sand or with all new sand, then, yes, that will mean a cycle is happening. Lots less likely with LS to happen, or, if it does, it will be min.