Sub enlarge_comment()
 'assumes that the current selection contains just one comment!
 Dim newrange As Range
 Set newrange = Selection.Range
 Selection.Comments(1).Range.Copy       'copy text of old comment
 Selection.Comments.Add Range:=newrange 'create new comment
 Selection.Paste                        'add text to new comment
 newrange.Comments(1).Delete            'delete old comment
End Sub