I'm not saying your wrong. I did it mostly because I knew it was a surefire way to not mess it up. I'm sure filter only would be enough but I didn't want to take any chances. Hence why I did it.If you don't change out the filter media, there should be no need to save the gravel. The beneficial bacteria requires high oxygenation and therefore is predominantly in the filter media. The amount on the substrate will be negligible.