Jing is tied to TechSmith's Screencast.com. When you install Jing you must create a Screencast.com account associated with a valid email address (unless you already have a Screencast.com account). This becomes your "Default Account". If you have multiple Screencast.com accounts (such as for work or personal) you can associate whichever account you wish on a per-button basis.
Should you decide to get Jing Pro, it's your Screencast.com account that is tied to Jing's "Pro-ness". In other words, if you bought Jing Pro and signed up with the Screencast.com account, "example@mail.com", and then change the Default Account in More > Preferences to an account that is not associated with Jing Pro, Jing restarts and reverts back to the free version. Re-enter your Screencast.com account you used when you bought Jing Pro to get the Pro features back.
See Also: Learn how to manage or cancel your Jing Pro subscription.