This is the problem when searching by String or Memo fields. It's easy to follow the history of field/menu workflows because it's a linear process that follows a timeline. No such thing with freely entered data in a string or memo field. If you have permissions to create a comment, then you have access to overwrite or delete previous comments. This is a management task and training issue. You can run Defect Reports that capture Comments on whatever time basis you want, but comparing a 'before' and an 'after' is tedious. The whole thing is like finding a needle in a haystack. Better instead to have the PM or QA Manager discipline the team to enter comments without altering existing comments. This is a people issue that cannot be scripted automatically into a solution instead.