Honestly if it were me, and I've done this before, I'd fill the tupperware container with their tank water, get the new tank set up, but leave some room for part of their old tank water and use it in the new tank and if you don't have a ton of fish, or really delicate ones, I'd put them back in the new set up. But you will have to monitor the ammonia/nitrites daily and do WC's if it climbs above .25. With using the same filter you will have a good jump on having BB in the tank. So if you monitor you water and do your WC's as needed I think you'll be good. This method has worked for me in the past. Just be sure when adding water to the new tank that it is really close to the temp water they are in.