I would remove the fish and shrimp first so they dont get hurt in the process. And I suggest keeping the old gravel in a container or a bag in the tank for a few weeks to help remedy any chance of minicycle.
I did a 100% water and substrate change (actually we boiled it instead of replacing it) with a 50 gallon tank. The three residents (all large algae eaters) did just fine in a bucket for over an hour, though I did have a heater and thermometer attached.
In your case if the water is fine, your tank is so small I'd probably just keep at least half the water and put it back in after you change the substrate. Should be a very quick job with such a small tank and that should prevent what somebody upstream called a mini-cycle.