The present disclosure is directed to a system and method for controlling and/or upgrading aftermarket multivendor wind turbines. The system includes a turbine controller configured to control operations of the wind turbine, a safety device configured to provide a signal indicative of a health status of the safety device, and a secondary controller inserted between the safety device and the turbine controller. The secondary controller is configured to receive the signal from the safety device over a communication interface. As such, if the signal indicates a normal health status, the secondary controller is configured to send the signal to the turbine controller, i.e. maintain normal operation. Alternatively, if the signal indicates a poor health status, the secondary controller is configured to adjust the signal based at least in part on a signal bias to an adjusted signal and to provide the adjusted signal to the turbine controller.