I told someone to be careful about changing their gravel because it holds much of the bacteria, and I got #### all over. Sorry to be blunt, but it's true.
Basically, make sure you have a big filter. There is probably enough bacteria in the filter to sustain a lightly stocked tank. And also make sure you clean the gravel that you are removing before changing it; I know that sounds pointless but if you don't it will throw debris from here to Tokyo, and therefore reduce oxygen levels in the water, and maybe bring predecomposed matter to the surface, resulting in an ammonia spike.
Before doing this, capture the fish and put them in a bucket. Move out all decorations and plants, and sieve out the gravel with a net, or strainer. Be careful to support the net though; the net method works well, but the net can be bent from the weight of the gravel.
-Lynden