RESOLUTION:
Unfortunately the Migration Tool does not update specific fields at the moment.
- The option: 'Replace existing item' setting will delete the existing one first and create new one.
When replacing an existing item, the old item is discarded, and the new item is written.
- The option "Add or upgrade list fields (if needed)" refers to the list definition
e.g. adding a new field in an existing SharePoint list.)
So it is better in this scenario to just map all required fields and re-run the migration job.
