My site is 8 years old. Basically I did implemented mod_rewrite, which then meant googlebot crawled thousands (eg 5000 per day) of pages, mostly duplicate content. About ten days later I noticed the drop in ranking for a 1 word search term of about 30 spots - ~30 to ~60. Other search terms weren't really affected. I then excluded those pages via robots.txt.
Changing URLs will drop your site in the rankings. It seems to me that it would be better to redirect from the old to new with 301s, not block the old URLs with robots.txt.