The UI does not allow you to deactivate a trial license in order to activate a purchased license under a different Telerik.com account. See attached screen shots for examples.
Telerik: we have implemented for this case a LogOut button on the Startup screen of Test Studio, which allows the user to logout from current trial account and provide credentials for another account, which has a valid purchase. The implementation will be available in our next product release 2015.1 planned to be out by the end of March 2015.