There is an article on the forum >>here
<< that will help you.
You can either change it all at once or in stages. In any event there may be a slight algae bloom but there should not really be any spikes in the water if you transfer the bacteria back. Using a new pair of nylon stockings, cut them into short lengths and tie one end. Fill with the old CC
from the tank and tie the other end. Bury the CC
bags into the new sand and in no time it will colonize the bacteria and some of the fauna (if any is there).
You may get some cloudiness from the new sand but it's nothing to be concerned about nor will it harm anything. Just remember to turn off a majority of the flow while doing the procedure to reduce dust storms...