In Quality Center, you cannot switch projects while logged in. To change projects, you must log out and select a different project during the login process. This is by design to maintain data integrity and session isolation.
In HP Quality Center/ALM, the project context is bound to your login session — switching to a different project requires logging out and logging back in (selecting the new project at the login screen), rather than switching projects on the fly within an active session. So the statement that you can change the project after logging in is false.