Hi ABB Rob,
Take a look at saving the custom control as a "Strict Type def". Then replace all of the occurances of that control with the strict type def. Once you have done this, updates tath are saved to the control will be applied to every occurance.
Note: If you pop-up and create a constant, it will be created based on the current strict type def. Changes will not propogate to these constatnts. Instead of doing the create constant thing, navigate to the strict type def. This will create the constant and link it to the strict def.
Ben