I currently have crushed coral for a sandbed and want to switch to sand. can i do this with the fish in there or do i have to set up a second tank, move the fish, remove the cc, put in the sand and then return the fish?
No, not whilst the fish and live things are in the tank.
I did exactly the same thing and it totally spiked my tank and it was wipeout. Had to start all over again. When you move the substrate you will disturb shed-loads of ammonia.
Hope this helps,
Tim.