Hi,
If the 424 pages are important and have specific content, you need to redirect all the content. You can't loose anything.
If the content isn't specific, You should plan to remake new pages with more specific content and take care about all the SEO factors (check out all the SEOmoz tools).
Use Webmaster tools to comunicate google the new sitemap. Now Webmaster tools detect if you're using WP or other software.
I did the same the past month, but with a big website to HTML5 code and redirect all the content. The pages load faster and Google crawler reach more pages. In 2 weeks the web had the same positions (but with google penguin update I had a little Google dance contest)
Check your top pages and content and be careful with, plan all your content and menus (architectural page content) and check all on-page SEO factors.
http://www.seomoz.org/blog/an-seo-checklist-for-new-sites-whiteboard-friday
Hope that helps ang good luck