Transitioning from .html to WordPress, SEO Concerns

Hello

I’m in the process of replacing my static .html site to wordpress platform. The current site has literally hunderds of static html pages. My main concern is that since i can’t use the html extension and since i will use the /%postname%/ structure in permalinks what will happen to those pages?

I was wondering what can i do so that i don’t use those html pages that have significant page rank when i complete the migration?

Any assitance is greatly appreciated.