What is the risk involved to single instance appointments if you select to let the exchange tool modify the appointments, KB article 930879 Options for updating mail table
If you let the Exchange tool modify the single instance appointments there is a greater chance that the appointments will be incorrect. Single instance appointments and meetings are not stamped with time zone information. So the tool doesn’t know if the appointment or meeting was made with the new or the old DST rules, so it assumes old and therefore moves the item. By rebasing soon after the OS update, you minimize the number of meetings that are created under the new rules that will get moved incorrectly.