You could maintain the bacteria culture by removing the old gravel, but leaving in the dirty mulm that would remain behind after you sift out the gravel. Then you could add a small handful or a sifting of peat to that mulm and put the Shultz stuff on top. You would lose the snails unless you made the effort to find them in the gravel and save them or a significant number of them. Shultz aquatic soil is pretty light stuff, so it actually might stay on top of old gravel even if you left it in there. But, in either case you would not lose the bacteria.