I designed a new theme for a WordPress website, but when I logged in wp-admin I got the worst surprise ever: version 2.3.2 was installed. It's such an old version of WordPress that I'm really wondering if upgrading will work. The website's owner doesn't want to lose any of the website's content, but the database is not compatible with 3.5.1. What should I do? How can I safely upgrade AND keep the content?