When rescheduling a calendar item (changing the recurrence rules) Outlook gives you this message:
Any exceptions associated with this recurring appointment will be cancelled. Is that OK?
You get this message because you have an existing recurrence schedule, but you could have changed the date or time of one of the appointments in the recurrence rule. Consequently one of the calendar items in the series might not match the recurrence rules. So when you change the recurrence rules, it makes you aware that the rules for all (including past appointments that are associated with the recurring appointment, but are exceptions to the rule) will be changed.
Unless you require a historic record of any previously changed appointments associated with the recurring appointment, it’s safe to click OK.