As noted in Migrating OTAA Devices from V2 to V3 one of the critical steps in performing the migration of devices is to trigger a new OTAA on the device, so the device receives a new device address / session keys and new network parameters from the TTN “v3” stack. A new OTAA is done for these devices after a reboot or power-cycle.
I have a couple of TBMS100 (motion) and TBHV100 (temperature/humidity) nodes deployed in a location where I cannot easily access them to reboot them by means of a power cycle. So I asked Browan support if there is some other way, like a downlink command to trigger a reboot. I got a friendly reply explaining that this does indeed exist.
You can reboot them remotely by sending the following:
unconfirmed downlink with payload 038000000100 (hex bytes) on port 222
Obviously this will not reboot the node immediately, the download command is scheduled just after the next uplink of the device, which typically happens once an hour as far as I know.
Before you trigger the reboot, you need to make sure that the device has been registered in the v3 stack and OTAA effectively disabled on the v2 stack (e.g. by invalidating the app key), as described in the link above.
I have so far only tested this on a TBMS100:
but I suspect this works accross the entire range of devices.