I exported a CSV of the tasks in a particular project, added some rows to the CSV with additional tasks, and updated one of the existing tasks (added a few words to the Notes).
I imported the saved CSV. All the fields (Description, Assignment, Due Date, Notes, Completed, Identifier, Parent) mapped correctly. The updated task had a value in the Identifier field; the new tasks had blank Identifier fields.
The new tasks were added to the project as expected. The task I updated (I didn't touch its ID cell) was added as a new task instead of replacing the old task. Why didn't it overwrite/update the existing task with the same ID?