It depends on the architecture of the original website. If you have hard-coded HTML pages, all you can do is redesign them and manually create a new website using WordPress, for example. But if your problem is that you already use WordPress and your website pages have mostly static content, the first step will be designing a new structure and then redesigning your website using the Crocoblock suite and other plugins for dynamic content if you need them (such as WooCommerce, etc.).