See https://www.thethingsnetwork.org/forum/t/the-things-network-upgrade-to-v3/43256:
- Activation by personalization (ABP) devices, i.e. devices that have a fixed DevAddr and session keys, are personalized for a specific network, in this case, The Things Network V2. You should consider V3 as a new network. So if you want to keep using ABP and you cannot use over-the-air activation (OTAA), it is highly recommended to create a new ABP device in V3 and reconfigure the device with the new session. Note that the (default) RX1 delay has changed from 1 to 5 seconds.