Warning: Always back up your data before performing any data operation. See Exporting Backup Data or Export Data for more details. It is recommended to run a test with a small subset of records to ensure the operation was successful through manually opening and verifying that the corresponding records are correct in Salesforce.
This is a manual process and may take a fair amount of time but will prevent duplicates, ensure that records are updated with relevant information, and improve overall data integrity on import.
See also:
Import Data with the Data Import Wizard
Insert, Update, or Delete Data Using Data Loader
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