After a registry has been migrated, individual devices can be migrated using the CLI migration tool. This tool allows for devices to be migrated multiple times. Please find more information available here.
As documented in the link above the Migration Tool requires a terminal to stay open while the migration proceeds. This may be cumbersome if the migration would take a very long time. In that case do the following:
Start the migration tool running in a terminal as instructed in the link.
Once devices and or gateways start getting migrated type Ctrl Z to pause.
Type bg followed by Enter to resume the operation in the BACKGROUND. The operation will resume but will still output messages to stdout.
At this point you can close your terminal but the migration will continue.