1+ the sand issue. This is the biggest risk. Old sand tends to collect detritus no matter how well you maintain your tank. If you stir up the detritus out of the sand in your new tank it will cause a deadly cycle. Rinse your sand in old tank water before re-use --of course. ALSO, when removing rock, GO SLOW, being careful not to stir up detritus as you breakdown the old tank. Get the livestock out before you touch the sand. You can also pre-cycle new sand in another tank or even a plastic 5G bucket and use that then ditch the old sand.