Create a development environment that allows users to try out feature (and features of the higher-level packages) without needing to pay for two website. For example, if I already have a paid account and site, I may want to make a copy of that site and do major changes/upgrades to it before making it "public". I may want to understand and experiement with how the customer account features work. Once I've made the changes and have it all working, I would redirect my URL from the old site to the new site. And the old site would no longer be active.
The development environment could have an obtrusive watermark, disabled transactions, password protected, hidden from search engines, weebly subdomain only, etc.
And this is a great way for customers to learn about more advanced features in the the higher account levels. Once they build in the feature, they will understand it and be more apt to upgrade.