I have my iPad and iPhone 5 "linked" to this. When I change devices, I turn off the bluetooth feature on the device I do not want it paired with. Then the other device syncs up. Then if I need to use another bluetooth device, turn it on, and can sync up that unit as well.
I have all four family member phones linked. I simply unplug the Bluetooth power source (seems to clear the last phone that was used) then go to settings on the device I want to listen to and hit connect. It works even if all four phones are in the car. That way, I can leave all the blue tooth on in case I want to use it for something else (like my polar heart rate monitor).