You will need to upload 301 redirects in your .htaccess file for all pages that you are replacing with new WordPress pages. You can put this at the end of the file. This will tell Google that your page has moved, to where, and to retain the current Page Rank. It also allows people searching for your site to hit old/indexed links, and be sent to the new ones, until Google's bot has the chance to re-crawl your site.
This portion of your .htaccess file will look like this:
You don't need to include the domain name if it's staying on the same domain. The old and new URLs should maintain their folder structures.
There are other ways to do 301 redirects as well, so give that quick Google before you proceed if you're looking for something more complex.
Finally, you should always use Google Webmaster Tools to let Google's bots know (sooner) what's going on with your site, and submit a sitemap to it: https://www.google.com/webmasters/tools/home?hl=en