Considering your possibilities:
1、The first base counter overflow or underflow after changing the RCR: This is correct. The new RCR value will be reloaded into the repetition counter at the first subsequent update event, which happens when the base counter overflows or underflows.
2、When the current repetition count downcounts to zero after changing the RCR: This possibility is not accurate. The repetition counter does not directly affect the update event itself; rather, it determines the number of consecutive overflows or underflows required before triggering an update event.