I did a product import with behaviour REPLACE using the backend.
The original reason to do this is, that we get duplicated images when importing a CSV file with same products again.
If I understand the documentation correctly, this does replace the full data of an SKU which is in the import file. But it leaves products with SKUs which are not in the import file untouched.
If a SKU in the import data matches the SKU of an existing entity, all fields, including the SKU are deleted, and a new record is created using the CSV data. An error occurs if the CSV file references a SKU that does not exist in the database. You can Check Data to display error.
In as test with only two products, this turned out to be true -- not all other products were lost.
But on my recent - real - import it showed: