Replying to myself: I got a friendly answer from Browan support.
There is indeed a downlink command to trigger a remote reboot of the node, which also performs the OTAA join again. I’ve tested this and it works, I see a new join, with an OTAA performed against v3 and the device receives a new device address, data coming in, etc. This mechanism can be used to migrate nodes from v2 to v3, as described in Migrating OTAA Devices from V2 to V3
specifically if physical access to reboot each sensor is difficult.
I’ll post about this in the https://www.thethingsnetwork.org/forum/c/v2-to-v3-upgrade/migrating-devices-to-v3/104 category later today or tomorrow.