I've seen some people claim that sand has a limited window of chemical activity. Something to do with buffering pH and contributing calcium. Obviously, I didn't catch all of the details. Anyway, these people say that you should add new sand, or replace part of your old sand bed with new sand, every year or two. People who believe this would probably advise you to keep a cup of the old sand to re-seed, but ditch the rest.
Others, of course, say that they've used the exact same sand for x years, and everything is fine in their tank, so you don't need to replenish or replace your sand bed. These people seem more likely to tell you to continue using the old sand.
As with everything in this hobby, eventually it seems to come down to which expert you're willing to believe, based on your own knowledge and experience.