I changed my gravel over a few months ago. I had this crummy stuff that was literally disintegrating, so I replaced it with small river rock. To do so, I removed 50% or so water into a large container that housed the driftwood, plants, etc as well as the fish (in my case it's only a 20G tank, so I used two 5G buckets). I put the filter cartridge in as well to ensure that the bacteria stayed wet. I then emptied the rest of the tank, scooped out the gravel (be careful, especially if your tank is acrylic, as it's *really* easy to scratch it while doing this). I scooped about a cupful of the old gravel from the back of the tank into a clean mesh bag, which I placed in the container holding everything else. Be really careful if you have ornaments, though, as if you are just placing things in the container there is a risk of things falling onto your fish. Also, I have shrimp and those guys are jumpers, so be careful!
Next I dumped in the new stuff (I had previously rinsed it well), filled up with fresh water, let it settle out, and then put the old water + fishies, etc back. I noticed a microscopic spike in the ammonia (and it might even have been me worrying too much), but other than that everything went smooth!
Of course, my tank is much smaller than yours, which shifts the logistics a fair bit... but hopefully this is some help!
Good luck!
