In regards to both of your questions, the switches or the network itself would revert back to whatever previous configuration was configured whether it be the default Meraki configuration or another config. There are no specific "gotchas" to be aware of but there will be downtime when moving the templates for the devices to pull the new configurations and if any subnets are to change if any static devices were programmed they would need to be reprogrammed under the new subnet.
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