You can just put a thin layer (about 1/4 to 1/2) of sand. It still makes it easy to clean like a bare-bottom.
If your display tank has the gravel in it, you can remove it slowly as to try and not stir up to much stuff. I would leave your filters on while doing this so they help grab some of the loose debris. Some people use a shop vac. I use a small dust pan (virgin dust pan/new) when moving gravel. You should not hurt your fish if done slowly. Before adding the fish, check your water parameters about an hour or two after you changed your substrate.