Removing the existing sand will cause some die off. There is no way I know to avoid that.
I believe the best way to covert is to remove all animals and drain to just a few inches above the desired sand depth and scoop out a few cups of the sand. Set this aside for the moment. Add the new sand right over the old sand. Distribute the removed sand over the top and gently refill the tank. Carefully monitor ammonia and nitrite levels. There may be some die off, but the sand and rock should be able to cope. If not, be prepared for large and request water changes for a few days.
I'm not sure what your specific reasons are, but if you're simply hoping for the benefits of a dsb
and aren't trying to change the aesthetics, consider a remote dsb
. This can even be done with a five gallon bucket for a sump, filled halfway with sand.