Brilliant spinner Daniel Vettori and warhorse seamer Kyle Mills will almost certainly step away. Those are McCullum's two closest friends in the team but his own case is a little more complex.
McCullum is definitely well advanced in his thinking beyond cricket. He wants to spend more time with wife Ellissa and three children an has moved into the thoroughbred bloodstock business, but he is also acutely aware of his responsibility to the Black Caps.
The astonishing feat of 100 consecutive tests shines like a beacon for McCullum. He has 92. It is hard to imagine him walking away before then. But ODIs may be sacrificed along the way.
You can bet your house on the fact he won't go on the three ODI, two T20 tour to South Africa this year, but I think there's a few more ODIs to come before he properly hands the reins to Williamson.