Hi,
I would change the Wordpress settings and setup a 301 redirect from non-www to www using htaccess (if you're using Apache, see e.g. http://enarion.net/web/htaccess/redirect-www-and-no-www/ for instructions how to do this). Updating the sitemaps file and resubmitting it to Google Webmasters, and manually fetching the www-version of the website with the Google crawler in the webmaster tools would be my next steps. Google and the other bots will use this signals to update their databases.
Best regards,
Tobias