Couldn’t find this in the docs or the forums, so I’m reiterating what this page documented here. I had the wrong baudrate and rtscts when I switched from my Sonoff adapter. This is what you need for the ZBT-2 and Zigbee2MQTT:
serial:
port: /dev/serial/by-id/usb-Nabu_Casa_ZBT-2_(your serial #)-if00
adapter: ember
baudrate: 460800
rtscts: true
After this is set, it comes right up and imports your existing devices smoothly if your old adapter was on ember.
FYI after a few failed attempts of much more complicated procedures and backup reverts I found a way to change from a zstack (electrolama zzp) to ember (HA zbt-2) coordinator using Zigbee2MQTT without re-pairing all devices. I don’t really know what I am doing, but perhaps this will help somebody else who gets stuck like I did.
back up zigbee network (Zigbee2MQTT web interface > Settings > Tools > Request Z2m backup > Download backup)
back up home assistant (just in case, then can revert later if need be)
Stop Zigbee2MQTT add-on
Rename coordinator back-up (via file editor) from coordinator_backup.json to coordinator_backup.json.old
Unplug your existing coordinator and plug in the ZBT-2 into a black USB-2 port on your HA controller, follow the autodiscovery in HA and select Zigbee > Other to get a firmware suitable for Zigbee2MQTT
Update configuration in Zigbee2MQTT add-on settings. You can find the serial ports via HA Settings > System > Hardware > All hardware, search for “serial” and find the one that contains ZBT-2 in it. The serial part of mine looked like this: serial: adapter: ember baudrate: 460800 port: /dev/serial/by-id/usb-Nabu_Casa_ZBT-2_9C139EAB357C-if00 rtscts: true
Start Zigbee2MQTT add-on
When I did this, all my devices were moved over to the new coordinator in Zigbee2MQTT. I had a reboot in there somewhere, so if this doesn’t work the first time try rebooting after step 7. Check the logs in the Zigbee2MQTT add-on to see what else might be going wrong. User experience might vary a bit depending on which coordinator you are starting from.