I’m sorry, I don’t think I understood the question.
The way the plugin is supposed to work is when the ID column is filled in it will try to match the ID. If the ID isn’t found it will create a new record.
For example, if your database has IDs 2 and 3. You import with IDs 2 and 4. The plugin should update ID 2, and create the new ID 4. After the import you should have IDs 2 (updated), 3, and 4.
Is that not what is happening? Or did you want it to do something different?