Yes, it is possible to change out the gravel with the tank still in operation, but it's very tricky. 1st make sure your new substrate is well rinsed, otherwise you will foul your tank. 2nd, it is possible to throw your tank back in a cycle when removing and adding substrate. Keep a test kit handy and be prepared to act should you get spikes. I would try to keep a small layer of your existing gravel in there to help prevent this. It is possible to scoop out all your gravel, lay down 2-3" of eco- complete and then a layer of your existing gravel (if you like the look).