If no place to put it, just take the old filters and throw them on the tank for quite a while and have the other filter running as well. Then after a reasonable time, remove it. Also as you stated putting chems in that remove chlorine, chloramines, etc will help. Keep in mind though if you do switch all of it over immediately after adding water, there will be a ph swing as the co2 in water hasn't dissolved yet. That's why people age water. To be safe I'd add the water, add the chems, run your new filter to turn over the water for a day or so, then switch the old filter onto there and then add fish.