There are a few patents from Google about how they deal with varying forms of content. There is content that is marked as static and then there is content marked as dynamic. Both are treated differently. Certain sites have a profile where content is continually updated, such as news sites, they will be indexed more frequently and under different guidelines.
Sites that are static, or pages that are static, usually fall in a more scheduled routine that is not as hectic as those sites that are flagged as "constantly changing". Let's take a blog for example, the home page is dynamic, usually. It will get tagged that way. The /archives/ are usually static and will be marked that way.
If you have specific pages that are static in nature, you typically don't make major changes to them. When you do make "major" changes, you may end up changing the meaning of the page. You have to take into consideration all the inbound links, where they are, what surrounds them and how that has affected your static pages to date. If you've been fiddling with the "meaning" of the page too much, you are creating an effect that trickles downstream quite a distance.