It sounds like you want to be able to reset the value of the Case Operating Temperature input field of a Refrigeration:Case
object instead of that input field being a fixed setpoint value. Normally, you would use EMS to do this. However, EMS allows you to override a pre-defined list of items in the model -- not everything in the model. These items you can override are called EMS Actuators, and the only actuator related specifically to refrigeration is the Minimum Condensing Temperature of a Refrigeration:System object. Sorry, but it sounds like you are out of luck on this one.
If you feel that this should be a feature of EnergyPlus, you could make an issue on Github where the source code is managed. Perhaps adding a new input field for Case Operating Temperature Schedule could be used for those who want a dynamic way to set this parameter.