Your sand bed would becomer populated with stuff off the live rock. A deeper sand bed may be better than a cc bed, but deep beds are not a cureall as they can and will harbor junk like hydrogen sulphides. Unless you have the critters that are capable of keeping a deeper sand bed stirred up I would not go with a deep bed. I fyou mix CC and sand, the CC will in short order all migrate to the top, and it will not stay mixed. Just place some small pieces of LR rubble inthe sand and see how long it takes to come to the surface of the bed........just a few days.
I would get some sand, wash it well, rinse it in saltwater (used is fine) If there is nothing in it thats organic, then the addition of new sand wil not initiate a cycle that would amount to anything. I konw of two individuals that had CC and changed to regular old play sand and what they did was use nylon mesh laundry bags to place CC in. They made a sand bed of playsand and then placed laundry bag of CC in tank for a few days. This allowed time for critters etc to migrate to sand bed. They swore it worked, I can't say it did or did not. You can rinse your CC as its removed with salt water, but your still gonna loose some stuff, but overall you will stil lbe eliminating the junk that normally builds up in a CC bed and not introduce it to the tank, and it is still going to be reseeded once rock is placed back in the tank. I think I would rather start off with a new dead sand bed than reinstall a CC substrate bed..Thats just me though as I am partial to a "shallow" sandbed of 1 to 1.5" as I do not keep anyhting that requires a deep bed so I eliminate the potential for any problems normally associated with deep beds. I got rid of my last deep sand bed probably 5 years or so ago and have not noticed a thing different in any of the tanks.