Honestly, I wondered the same a little while back. I wanted to change around one of my sites, but finally decided only to make minimal changes. With Google's alogorithm constantly changing, I didn't really want to risk making my sites rankings plummet. I'm just saying to be careful as to what you change. For example, if you choose to target different keywords, and not target the previous ones any longer, as popcrdom29, your rankings will likely drop pretty fast. If you're simply targetting more keywords, I don't think it'd be a problem.