To switch a transmitter from one LoRaWAN network to another, you do not need to physically intervene on the transmitters. You just need to remove them from the old LoRaWAN network, then declare them on the new LoRaWAN network.
Indeed, every 24 hours (37 hours for the US915 products), the transmitters send a LinkCheckReq message which must be confirmed by a LinkCheckAck by the network. As long as LinkCheckReqs are confirmed with a LinkCheckAck, the transmitter will not send a new JOIN. If the transmitters are still declared on the old LoRaWAN network, they remain connected to it. On the other hand, if they are declared on the new LoRaWAN network and if they are no longer provisioned on the old one, the failover to the new LoRaWAN network takes place.
*JOIN: this is a phase of access to the LoRaWAN network which allows to dynamically renew the network parameters between the end-device and the LoRa Server.