I removed all of my gravel last year. I lowered the water level. I pumped some of the water into a 30 gallon Rubbermaid tote with a heater and airstone. Then I removed all the gravel and cleaned the tank. I replaced gravel with sand. I filled tank and let run for several hours before adding fish back.
You will want to monitor levels daily to be sure you don't have a mini cycle and change water accordingly. I dosed stability just in case.