Reimport existing constituents by contact ID

For GiveCampus constituent data and imports.

Yes, when the source Contact ID is mapped to the constituent unique identifier consistently within the same school, the import matches that identifier and updates the mapped fields instead of intentionally creating another constituent. New identifiers create new records. Check transformations/padding and a test sample first; changed IDs, different record types, blank handling and append/replace rules can affect results. This is not duplicate matching by name alone.