Often, when new sensor or actuator hardware be- comes available for use in a control system, it is desirable to retain the existing control system and apply the new control capabilities in a gradual fashion rather than decommissioning the entire existing system and replacing it with an altogether new control system. However, this requires that the existing controller remains in action, and the new control law component is added to the existing system. This paper formally introduces the concept of Plug-and-Play control and proposes two different methods of introducing new control components in a smooth manner, providing stability guarantees during the transition phase as well as retaining the original control structure. The applicability of the methods is illustrated on two different practical example systems, a livestock stable climate control system and a laboratory-scale model of a district heating system.