Ordinarily I would say don't worry about replacing the sand, because it can go literally years before it needs replacing. However, in your case, since it's a pool you bought and don't really know the history of, and since you're going to have to remove all the sand to check for lateral cracking anyway, it might not be a bad idea to go ahead and replace it. Not only to make sure you don't have residual crud in it from the previous owners, but also to establish your own history with the pool and to make sure that you have the right amount of sand in the filter from the beginning.

Just my $0.02

Janet