You can upgrade any time. Pro uses the same structure and class names as Free, so the pages you've already built keep working.
- Buy Pro and download the files from the link in your receipt email.
- Copy the Pro
css/andjs/folders over the Free ones (your owncss/custom.cssisn't touched). - Add the Pro pages, apps and dashboards you want, and link them in your sidebar.
That's it — there is no license key to enter and nothing to activate.