What counts as "new content" exactly? If I have a site that has www.site.com/snakes.html without much content, then later add good content to it, will the SEs realize it's different?
Usually it's beneficial to add new content, especially if it's related to the already existing content. But you probably should do it gradually, one page at a time, rather than a large number of pages all at once. If you want to change the basic theme of the site, that should also be possible, but again you should do it gradually by slowly deleting the old pages and and slowly adding new pages.
Your server will tell the search engine bots when pages were last modified. If your server clock is set correctly the bot will know to re-index pages that have changed since it's last visit. You can also submit a google sitemap and tell the bot how often to expect changes.
That's good. I was thinking this, because http://music-nerds.com has several hundred pages each featuring a different CD. Now, many have just track lists, album art, and that's about it. Slowly, in-depth, original reviews are being added to them, but the URLs don't change. I need the bare-bones ones up though because that's how people know what albums are still available to write for. It makes sense that a good XML sitemap would tell the SE that the content is new. Thanks!