Unfortunately, the two stores are completely different softwares and thus there is no upgrade path. What many people do is to maintain their Sitebuilder site and design the Storefront separately. When ready, you change the store link from the Simplestore to the Storefront. This means you have both a Sitebuilder website and a Storefront Ecommerce site, tied together. You still have to build the Storefront separately.
The Storefront professional is $29.99 per month and can standalone as your entire site. If you want to maintain the Sitebuilder part as well, add $4.99/month for up to 5 pages. You can keep the site as is while you build you Storefront as well, so that people can be buying from your Simplestore while the Storefront is being built.