If you will have the same URLs, then there is no issue at all. Google doesn’t care if the pages were created by WordPress or by any other CMS, or by hand for that matter. If the content comes up on the same URL, then Google will continue to give you whatever credit they currently give you for those pages.
If you are moving to new URLs, you simply need to create a 301 redirect from the old URLs and Google will pass the “juice” along to your new URLs. A 301 redirect tells Google that you have permanently moved the page to a new location and updates their records accordingly. It will take a couple of weeks for Google to fully make the transfer to the new URLs however, so if you have rankings, they might disappear for that time, but will be right back in after 2-3 weeks.