Minor update: I was having occasional issues with my Zen's not changing attributes via automation and my units were both on v2.16 of the Zen FW. As there wasnt a way to update them via HE, I bought a 2nd hand SmartThings v2 Hub for $50 AUD and used that to update them.
The process is quite easy and you dont need to break all your automations and dashboards to do it.
- Take your Zen off the wall, wake up the screen if it isnt already on.
- Press the network button 10 times to remove it from your HE Zigbee network - this is only done on the device, your HE device won't be removed unless you run that process on the HE (so dont).
- On your SmartThings app, select add a device and pick the Zen Thermostat
- Press the network button 5 times to add it to your SmartThings
- follow the instruction HERE to check your Device FW version
- pull the batteries out of your Zen for a minute
- Wait for up to 10 mins and it should auto-update - you'll know it's doing this as a
symbol will start randomly flashing around the screen. It's done when the display goes back to showing "prefs". - make sure the Zen is responding in the ST app then power down your ST HUB
- Press the network button 10 times to remove it from SmartThings
- On your Hubitat Hub, go to discover Zigbee new device and start the discovery process
- Press the network button 5 times to re-add it to your Hubitat - you'll see it say something along the lines of "found an existing device". This means you succeeded! Congrats!
Hope folk find this helpful. 
