I guess it would depend if you are going to have any diggers in the tank...
When I did my 24g to 75g upgrade, I used my existing rock and also added new rock. I first put the new rock in (touching the bottom), and added sand. Then once the tank stabilized over a week or so, I added my existing rock. With my existing rock, I just pushed it as far down into the sand bed as I could, as I had planned on getting a jawfish and pistol shrimp (both voracious diggers).
(BTW, Welcome to ARC! I just noticed that was your first post )