I did this exact same thing in December. When I did it I had been competing with an old Under ground filter (Ugf) for 3 years so I finally pulled it and all my gravel. I did have my tank water only 1/2 full when I added the ADA Amazonia soil. I can't even remember when I added more water. Let it settle for a few days after filling back up with water. It took mass water changes to get my parameters right. And then when they were right I added my sand as a topper. I didn't unplug my filter because all my fish were still in the tank.