Free hosting and premium hosting are completely separate from a technical perspective. They use different servers, different control panel and a different technical architecture. They are provided by different companies even! This does mean the upgrade isn’t entirely seamless, but it does mean you’ll end up with a true premium service at the end.
So if you want to upgrade to premium hosting, the way to do so is to order a new premium account at iFastNet, and then ask their support to migrate your website for you. They have tools to migrate your entire website to their premium hosting (including all domains, files and databases) so you don’t have to do anything for it.