If you just bag up some of the old substrate and put it back in with the new stuff, you won't have to worry about losing too much of your bb...
My suggestion would be to: pull your stock and large deco out and put them in a crate or two with tank water. Thoroughly gravel vac your existing substrate and pull it out (I prefer a dust pan to pull for this) and bag some of it up (ladies nylon stockings will work in a pinch). Then just add the new substrate you'd like to change to, do a large water change and add everything back to the tank. It may take a little time depending on the tank size but as long as you keep the deco and bagged substrate soaked and have a good amount of bb built up in the tank and filter(s) you shouldn't experience any sort of mini cycle. I've done this on a few tanks, and have never had a problem, just make sure to leave the bagged (old) substrate in for a week or so before you pull it out.