The paper proposes the mechanical design of a lockable spherical joint, which is designed to be manually or automatically configured in different kinematic solutions. In fact it can be used as a conventional spherical joint or converted in a universal joint or still downgraded to a revolute joint. Therefore different configurations can be chosen according to user needs. In particular, two of the three axes of revolution, arranged in the typical roll-pitch-roll sequence of robot spherical wrists, can be locked alternatively in order to provide two differently arranged U-joints. It can be demonstrated that such behavior allows to activate different mobilities of two classes of reconfigurable parallel kinematics manipulators. The transition between such mobilities occurs exploiting the concept of over-constrained kinematics, which is realized by the lockable joint during the switching phase in order to avoid an instantaneous mobility of the robot.