If your careful and can wiggle the old one out of the pad without breaking it, you can get away without replacing it. As long as the pad didn't wear till the sensor turned on the brake pad warning light. If that happened it's done.
If the light isn't on and you decide to re use it, make sure you get the little clip out of the pad, it will stay in the pad. Retrieve it, squeeze it a little to tighten it up and slide it back on the sensor. As long as your using factory pads it should slide right in. Aftermarket pads are too thick and don't seem to accept the sensor, they break when sliding into the pad