Additional info from Mike Miller on the resetting the control unit:

The X-Drive transfer case must be reset electronically at the dealership after the oil is
changed. You can change the transfer case oil yourself or have it done wherever you
want, but the vehicle does have to go back to the dealer to have the transfer case reset.
This “resetting” procedure is only possible on the BMW service computer.

Specifically the “reset” uses the actuator inside the transfer case to gauge the thickness of the remaining clutch material. The transfer case then relearns the end stops of the clutch pack to compensate for wear. The electric motor inside the X-Drive transfer case then knows how much it has to apply the clutch pack in order to transfer power when
necessary.