Set up the new tank using as much of the old water as you can, and put the old filter media inside the new filter along with the media it came with. (Unless you can replace the new media entirely with the old stuff. Depends on the filters.) If it won't fit, run both filters for a while (a month?). There is a possibility the new tank could mini-cycle, especially if you're changing gravel and ornaments, but it shouldn't be anything to harm your fish. Be sure to test the water daily for a while after switching.