Alot depends on the rock. If what lives in the sand happens to be on the rock and not dead yet, then you might be in good shape, but I wouldn't bother. Just get a friend to scrape off a nice big cup of sand from the top 1" of their tank, and put it on top of yours. I'm using 5" of SD sand and the last 1"..I got lucky and scraped it off the top of a 1yo tank that was being broken down, so I got ALOT of LS. Although almost all of it was SD sand seeded from somebody else. The tanks been up for about 6 months and is looking great, although the only corals I've put in are frags. You can see alot of worm tracks against the glass from where inhabitants have taken refug in my SD sand.
--Fizz