If you move the sponge, you're also moving the only source of bio-filtration so move it to the new tank. I assume the sponge is in a HOB filter or has water flow. Watch ammonia in the new tank closely and you should be able to move all the fish at once. A seasoned HOB filter will do a good job with ammonia/nitrite. Be sure PH, temp, and SG are about the same in both tanks and just toss the fish in. If there is much difference in these parameters, acclimate more slowly. I'd still be concerned about the cycle status in the new tank, if you keep the old one set up a while, you can always move them back if ammonia climbs. If that isn't practical, be sure to have an ammonia-neutralizer (Ammo-Lock, Prime, etc.) on hand and be prepared to do lots of water changes.