You can't, unless the main table remains always same, or you update your main table manually row-wise! In case the main table can change, and you must keep old comments, you must have those comments updated in another table.

Another possible solution will be having a separate table where you copy new data, and then run a VBA procedure, which compares both new and old data row-wise, and overewrites row-wise old data when there is some difference. What I adviced is much easier, I think!