Should I turn on maintenance mode during migration?
If you transfer data to the live store, it is recommended to turn on maintenance mode on your target shopping cart before migration. This action will help to avoid possible loss of the orders which could be created during the migration procedure. In maintenance mode your customers will see a temporary page with information about service works.
You can find the information on how to turn on maintenance mode in documentation for your shopping cart.
Here are instructions on how to enable maintenance mode in some of the shopping carts: