if you do use your sand, wash it really really well.
everytime I have tried moving sand without washing, bad things happened.
as everyone else has said, there is alot of stuff in sand that sitting in buckets for a few days won't get rid of.

But in all seriousness, this is the way to go. Whenever I transfer my tank I get new sand and add about a few cups of the old sand the "seed" it.

Just as a counter point to this, I know a guy on here who rebuilds his tank regularly and washes the sand to re-use it. He, however, is doing this with a 20G where it is much, much easier to do a really good job of washing the sand.

I have move sand from 2 - 65 gal tanks in to my 165 gal after washing, transferred ALL the old water & rock topped it off with new water to desired salinity, let it all settle for a day & in went all my fish & coral, didn't lose a thing.

Also did the same with my FOWLR. I think the key is to use the right thing to wash the sand, I used a fairly large fish net with very small netting. Washed a couple cups at a time in water as hot as I could handle.

Not going to say it's the right thing to do but it has worked for me every time I have moved a tank.

Twice now I've moved sand for upgrades. Sand was slightly washed and put into tank. Were it was filled with old water circulated for a few days then water change to remove all the crap. So far so good.

Correct me if I'm wrong but is the sand not from the ocean where the fish there poop on it there.

The risk in moving a sandbed is hydrogen sulfide from anaerobic areas. These look like black patches in the sand and stink. Release of Hydrogen sulfide will kill fish and inverts - and humans too if the concentration is high enough (shouldn't happen in a tank...not hobbyist size anyway).

I haven't personally transferred sand to a new tank but when I purchased my used set up I retained the sand from the previous owner. I was a newb & didn't know better, in hindsight I would have rinsed it well before restarting the tank. I went to Vancouver from Vernon to pick it up and needless to say it needed to be emptied for transport. Live rock went into a rubbermaid and was wrapped in newspaper soaked with tank water, sand stayed in the bottom of the tank with a little bit of water. Of course no livestock was involved in the initial set up so it didn't matter too much that I was a newb & fired up the tank with all this old stuff. In hindsight I probably should have given the sandbed a real good rinse. I think as long as you take a few precautions such as rinsing well and allowing things to settle before you transfer the livestock you should be able to use all your old sand. The old water is not going to do much for any kind of secondary cycle or transferring beneficial critters, the sand will likely harbour much more of the stuff you want to get the new tank going again.