Google placement is never guaranteed. If anyone tells you it is, they are trying to sell you something they can’t actually supply.
Google placement is a very complex algorithm and if you change content on a page radically it is definitely possible to lose your position.
The main factors in determining your position for specific keywords are – meta tags, page content, incoming links, and content on pages with links coming into your page.
I wrote a small blog about meta tags here that might be useful.
Abraham