Dear munders,
SEO and Page Ranking changes rapidly when the quality of the content goes down / 404 errors (NOT FOUND ERRORS) etc…
As long as your url’s are maintained as they were in your earlier site and content remains the same, It should be a problem in retaining the Page Ranking.
Also, you can resubmit your SiteMap using Google Webmaster Tools, so that your not found url’s will be replaced with new urls by google over the time.
At the same time i am no expert in SEO. I had changed content on my site and URL’s got changed too, as i deleted old content and replaced it with new articles, my Page Ranking fell from 4 to 0. It took me over 5 months to reach back to Page Ranking 3.
Arvind