GTM-Migration

Personally if you're able to do so I would set up the new GTM separately and add it to the sync group, this give you more flexibility to fall back if necessary and avoids IP conflicts. You can then update your NS records with your DNS provider with the new GTM listener address when you're ready to decom the old GTM. When you're confident DNS queries are not being served by the old GTM you can shut down the listener and remove the device from the sync group.