When a customer's Plume network setup is complete, the last step is always to turn off the old WiFi if their old router is still being used. This is a crucial step for many reasons.
If the network was initially provisioned or set up to make the transition of devices easier, devices should just connect to the new Plume network as soon as the old Wi-FI is turned off. Otherwise, the customer will have to setup every device for the new network.
The public Plume Support Help Center has articles which cover how to turn off the WiFi on many brands of routers. US / UK / DE