Bought new sand, but using all existing live rock, and existing sump. Tank will be in same room, only about 6 ft from existing. Specific questions are do I need to cycle new tank or can I just use as much of existing water, live rock .
You can just move everything over. You would want to monitor parameters for a little bit after just to be sure you don't get a mini cycle. I'd make up another batch of water to have on hand and be ready to do a water change. Ditching the old sand is smart IMO.
Whenever I do an upgrade or tank swap, I always end up having what I call a mini-cycle. Do you have corals is the current tank? As long as you keep the bioload the same, there shouldn't be a big issue unless you've got a lot of fish. You're losing a lot of beneficial bacteria and this can be supplemented by bacteria in a bottle. You'll still have some of the "new tank uglies" going this route so don't think something is really wrong when you start seeing diatoms or GHA. Tank swaps and upgrades are the only time I really ever test for ammonia on a regular basis and I've found the cheap API kits sufficient. If I were you, I'd also get one of Seachem's Ammonia badges just so you can constantly monitor it.