Any change you do, you risk changing the chemistry of the tank.
How old is the tank? Are you going to change all of the sand or put the black sand on top. Your crushed coral is live with beneficial bacteria, you take that away you risk a mini cycle.
As long as you leave the rock fully submerged in the old tank water, it should be fine. If you really want to risk this and you want to take out the crushed coral, than you need a bucket that can hold all your water. Put the live rock in the bucket. Than get another bucket and put some tank water in that and move your live stock too it. Once everything is out, take out the crushed coral and put your black sand in. Put the tank back together and turn everything back on.
Personally, being this is a Seahorse tank I wouldn't touch it. I would deal with the crushed coral. Seahorses can be really fragile! If you really want to, just get some black sand, and rinse it out. Get all the crap out of the black sand, and than just put it on top of your crushed coral. That would be the easiest way!! Don't move anything, just put the sand on top and move it into the crushed coral with your hand. Spread it around so it covers the top of the crushed coral.
Fish Sticks, the best thing you can do with your dead fish.