Upgrades have additional features to help improve your website, including extra pages, blog features, and finer graphic customization control. This also means it is not possible to downgrade to a lower version.
However, you can create a new free Starter site and quickly rebuild your website there. Then, if your original website uses a .simdif.com address or a domain name managed at YorName, you can give the address of the original website to the new one.