After spending two minutes clicking around my free account, I couldn't see either how I'd upgrade to a paid account or why I'd want to. Seems like an important usability issue.
Most of our customers run GitLab on their own servers, for which you have to pay if you want to run Enterprise Edition. You'd be prompted for a license key and the means to get one.
It might be a good idea to do this differently in the future, but we're not keen on running anything else than default GitLab EE on GitLab.com.
I'm not one of those people, by the way, but I do understand the frustration of seeing someone handing out cheese and crackers, and after stepping up to ask for "no cheese, thanks", being told they must take it with cheese.