Will I get penalized by Google if I rewrite all my content?

Many times its actually good to refresh your content because Google sees it as continual updating and adding value (as long as it isn't trash spam content). Are you redoing a site or just changing content? I ask because redoing your site and changing the URL's will impact your site & pages ability to rank if they change, as they will lose their "link juice". In this case you just need to 301 redirect those URL's to the corresponding new URL.