Are you planning to run a HOB
on the new tank? If so, I would move your 400 to the new tank and put the new filter on the old tank until the new tank cycles. You should have enough bacteria in your canister to keep the old tank going. You might take some of the gravel from the old tank and put it in a bowl in the new tank along with some water from the old tank. I used this process to upgrade from a 50 gak to a 125 and had no measureable cycle.