Hello!
I have my site since January 2010 and my pagerank is zero. So, I decided to move throughout the structure to improve SEO. I read some articles on google's own content and link structure and decided to change the permalinks. As my site is a magazine, I adapted the structure of links similar to the major magazines.
Before: /%year%/%monthnum%/%day%/%postname%/
http://www.clickcamboriu.com.br/2011/07/28/padaria-e-assaltada-no-taboleiro
Now: /%category%/%year%/%monthnum%/%postname%-%post_id%.html
http://www.clickcamboriu.com.br/policia/2011/07/padaria-e-assaltada-no-taboleiro-21593.html
My first concern was about the google search and 404 error because the search results of google, it still displayed the old links. This problem was solved by installing the plugin: Dean's Permalinks Migration.
Now no longer returns a 404 error, and enter the old link, the user is redirected to the new. But the counters of tweet, like, and +1 google button of each post were "reset".
Then ask the community, changing permalink I have done is beneficial or catastrophic? This can really improve the SEO, or actually change the structure of the links after so generated content (4350 posts) will only hurt me?