You can no longer change or delete day book lines once their corresponding transactions have been matched. To do so, you must unmatch the transactions first. Once journal entries have been matched, the 'Matched' flag and the matching code are added to the day book line (you can display the columns in the list of day book lines). In that case, the line is blocked for changes and deletion. When you unmatch the entries, the flag and matching code will disappear, and the day book line can be changed or deleted again.