6.3.3.

Why not always update RECURRENCE-ID? Since RECURRENCE-ID is our key to find that particular instance, it should not be changed unless the entire set is being replaced. The reason is best explained with an example: Chair sends out a 5 day recurring meeting, Monday thru Friday, from 9am to 10am. Chair later reschedules Wednesday to be from 1pm to 2pm. One of their recipients does not receive the reschedule for Wednesday. Chair reschedules Wednesday again this time from 3pm to 4pm. If the RECURRENCE-ID was changed during the 1-2pm reschedule, then the recipient will not be able process this reschedule or any subsequent reschedules or updates. That RECURRENCE-ID will never match any instances on their calendar.