If done properly, the only real risk is transferring an unwanted pest or two. But yea, you can effectively insta-cycle your new tank and transfer everything over in short order.
When I upgraded I seeded my new rock for a month or so prior to the arrival of the new tank (black rubbermaid trough, existing tank water and filter floss from water changes, heater, wavemakers, bacteria and ghost feeding).
Then I set the new tank up with fresh saltwater and sand, got everything running (filtration, heat, flow, etc.), added the rock I'd been curating and added an appropriate amount of Bio-Spira for the tank volume.
Transferred over my existing rocks (lifted them out and picked off any bristleworms), corals, fish and finally inverts (sifting through the sand to find a few stragglers). Then wound up my old setup at my leisure.