A single end device can only be registered in one network server at the same time. If it’s registered in multiple network servers the behavior is undefined, so don’t do that.
With the Packet Broker we now have peering between networks, so traffic received by one network (the public community network) but intended for another network can be offloaded to that other network (v3) and vice versa.
We still need to write documentation for the migration from v2 to v3, but it will probably look a bit like this: First update your application to work with data received from both v2 and v3. With a test gateway and some test devices on v3, you make sure that your application still processes data correctly. Then you can either use Packet Broker peering, or a poly forwarder on your gateways to get gateway traffic into both the old v2 network and the new v3 network. Then you can start migrating existing devices from v2 to v3. If you work with OTAA devices this is quite easy. You register your device in v3 and then change the registered AppKey in v2 (so that it no longer joins on v2, but keeps the current session). The next time your device joins it will be on v3.