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Choose "Remove Source Gateway" - to delete the source gateway's configuration from DPOD.
Even after the configuration is removed, you will still be able to see old transactions and syslog data from this gateway in DPOD (depends on your storage capacity)
Choose "Keep Source Gateway" to keep the source gateway in DPOD after the migration is done.
After the The Secure-Restore will be done, the target gateway will get process will assign the same source IP of the source, if address to the target gateway - if you wish to keep both - you will need to manually change the source's IP address and enter it in the input fields "New Source Gateway SOMA Address" and "New Source Gateway SOMA Port" of the source gateway.