A good bit of the good bacteria will go out with the gravel. What you can do is drain about half or so the water out into a bucket and put your decorations, filter pads and fish etc. into the bucket to keep them wet.
Then drain the rest of the water out. Now it will be much easier to take the old gravel out . Don't wipe the inside glass down, don't clean the decor there is bacteria growing there also. Put new gravel in after rinsing. refill with water add decor, fish, put the filter media back in the filte,r etc back where it goes. You could put some of the old gravel in a panty hose foot(or any net bag) to help the bacteria catch back up to the bioload. Watch your
ammonia for awhile you cold have an ammonia spike from removing so much good bacteria. since the water doesn't have much good bacteria in it it's not necessary to put it all back, Let the tank run for a bit ( an hour or two)to out gas(see the bubbles on the glass?) and warm up after filling it back up.
carol