FYI You should also know that if you are planning on adding a bunch of new content to existing sections of the site that new content will impact rankings. This is because the engines have to evaluate the new content to see what impact it has on the rest of the site in terms of link inheritance and overall contribution to the theme of the site or section. This will delay your site’s re-indexing. http://www.textlinkbrokers.com/blogs/more/216_0_1_0_M/ hmmm...
I can tell you, as the person who wrote this article, that I've seen this happen on a very well established site that's been online for more than 10 years. There were no other changes to the site other than adding the new section and they suffered decreased rankings and traffic for 6 weeks. I've also seen in on other sites to varying degrees. In each case an addition to the site affects it for a period of weeks. My intention wasn't to sell seo services, more to consult with your seo if you have one. And if not, at least ask a few questions before moving forward.