Site structure and page rank ponderings

[Crunch]

Hi I am rebuilding a decent size site with 2000+ pages and have fallen into rewriting all urls so they "appear" to be in the root directory. As in www.site.com/file-name-2000.html , the question what will this do to my PR6 site? The previous site structure was very directory sub directory based.
www.site.com/category/sub-category.html etc The home page PR6 then the next level PR5 etc. Anyone tested this lately, or have thoughts on this?

[incrediblehelp]

Why are you even changing the URLs to begin with? For PR reasons? If yes, then dont do it.

If it is because the search engines were having trouble crawling the old URLs then do it.

The history a URL has earned is so much more valuable than most chnages you can make to a URL for SEO reasons.