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Create a custom field as an External Id filed in each and every object that is going to use for data migration with data populated. Using the same field label for all the objects makes the setup easy.
It is a little difficult to populate the unique data into the new field. To make the process easy, you can create an Auto Number field first and after populating initial data into all existing rows, you can update the data type to Text.
Once the External Id is set in all the objects in the Prod Org, refresh the data to all the partial, and full sandboxes, and dev org to make it in Sync.
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