I've just moved a spawn of cichlid fry into a larger aquarium, about 120 gallons, from the 70-gallon in which they have been growing out. I filled the new tank with 100% brand new water (untreated well water) of the correct temperature, lifted the mature sponge filter out of the 70 and plopped it into the 120, netted out the fish directly into the new water. I've done this exact same thing dozens, maybe hundreds, of times over the years. It works without fail; I don't even bother testing the water anymore, just go right on feeding the fish as always. It's literally no different than doing a near-100% water change in the existing tank, which I have definitely done hundreds of times; you simply move the entire bioload into the new tank, along with the entire mass of mature biomedia, and carry on normally.
Even moving 50% of the media works. In that case I feed very lightly for a day or two, but that's about all the time it takes for both the new and the original cycled tank to ramp right up to full capacity. I've tested this enough times in the past that I don't consider it an experiment, it simply works...no matter how hard some people try to make it complicated, it isn't. I think the biggest danger is what
Backfromthedead
alluded to, i.e. moving the media and then starving the BB on it by not supplying it with food right from the get-go...and even that takes some time to become a problem, and very little time to correct itself.
