How to upgrade to a larger aquarium?

I currently have a 15 gallon column with some decor but no substrate. And I made a tiny upgrade to a 20 gallon long, it s all I could afford with out buying new items like a filter, heater, etc. This 15 gallon is cycling after an ick infestation killed my fish. Checked my parameters today and ammonia is .25 nitrite 5.0 and nitrate 5.0 (this is mid-cycle but I think it s almost done?). All the stuff I got for the 20 gallon was free but used including a fluval U2 (except filter media is new). So when I m done cycling how do I move all the stuff from the 15 gallon to the 20 without messing up and having to cycle AGAIN, I seriously do not want to cycle it again. I will be using the aqueon 10 (the one that is cycling with it) and adding the fluval as well with it. So 2 filters. Will adding another filter screw it all up??? Oh and this all a fish less cycle so I have no fish and won t be getting any until the 20 is done and ready.