This is what I would do. Get a 30g garbage can. Get enough for your tank water volume. Fill them up with water and let it age for couple of days with an air stone. You may want adjust the ph, gh, kh, and fert contents same as your tank water. Then, you turn off the filter. Remove all the plants and fish and put them in another tank or another bucket. Then, drain the tank water at the substrate level. Remove substrate. Drain out all remaining water. Put new substrate in and starting to rescape your new setup. Fill the tank with water from the plastic garbage can. Add fish. Turn back filter on. You may able to keep the filter running with water from the garbage can. When you are done, keep the lights off since your CO2 concentration is pretty low as this point with the new water and setup. Any additional light would promote algae growth. As long as your filter media has enough nitrification bacteria, you shouldn't need to cycle the tank again.