I would reset them. Use a paper clip to push and hold the reset button while inserting the new battery. When the LED lights release the button. After a couple seconds it should blink blue. When it does, go into the ST app and add a device as if it were new. After a few seconds to a minute, the LED on the contact sensor will flash green indicating that it has reconnected.
You should be fine after that. One thing that concerns me, you mentioned they were fine for 12-18 months. That’s a short battery life. I’ve never had any last under 2 years with most lasting at least 2 1/2 years. Short battery life could indicate a weak Zigbee mesh. The device may have to transmit at a higher power to communicate with its closest neighbor which definately impacts battery life.