I am guessing the current gravel is inert and so you want to take it out.
Safest way to do this is firstly try to save at least 50% tank water by siphoning it into buckets. Then put fish and plants into a bucket with some tank water (not too little water that the fish quickly pollute it). Then you are free to disturb the current substrate which probably has a lot of toxic nitrogen gas pockets under it. You can either take out the substrate right away if you dont care to clean it. Or you can stir it up in the tank to release the dirtyness and fill it with tap water, siphon the dirty water back out, fill it up again, empty water again, fill it again (repeat until water is clear/not murky), then remove old substrate. Or you could remove substrae and clean out of tank later. Add new substrate (not sure if non-inert substrate is still recommened to be rinsed first). Maybe do another quick water change if water gets murky from new substrate. Once clear/clean enough water, only fill about half way so you can fill the other half with the saved aged tank water. Then replant and then add fish. Walla.
You could do the switch while the fish are in their but that is much messier, takes longer and is much more riskier in exposing the fish to nitrogen gas releases.