overlaying works at first, but when you do a gravel vac, you'll probably end up mixing it up a bit. you could do the slow change method. remove a small area of the old gravel, and replace with new. only do this once a week or once every two weeks. you don't want more "unseeded" gravel than seeded (with beneficial bacteria that is). that way, the new stuff has a chance to build up the bacteria and eventually you'll have it all switched out. it doesn't look real pretty for a while but its a safe way to do it if you've already set up with fish and such. right now i have a tank thats half the old black and half the new natural gravel, takes time, but haven't lost any fish.