If you’re worried about having a cycle, you can replace the sand in increments. But that’s not the same thing as cleaning the sand in increments and then just putting it back into the tank. The sand is now loaded with nutrients and putting it back into the tank will just release those nutrients back into the water. I hope you understand this now.Vacuum the sandbed removes the surrounding detritus but leaves the beneficial bacteria that grows on and in the sand behind. Replacing the sand will cause a drop in beneficial bacteria which can cause a cycling event.