I found some documentation on the UNO beta, which tells me it is using the Microchip RN2483 (with embedded LoRaWAN Class A protocol) which also offers:
mac join otaa
Likewise, version 1.5 of the IBM “LoRa WAN in C” offers LMIC_reset, LMIC_startJoining and LMIC_tryRejoin (with the latter the session to the current network is kept if no new network is found).
So, assuming that for the RN2483 mac join otaa also works if a different network was already joined earlier: all this seems easy enough, if the node is programmed to execute this after it has not received any response from the application.