The ability to control which nodes are deleted when a MachinePool is scaled down is controlled by the spec.deletionPolicy as described in https://docs.openshift.com/container-platform/4.14/machine_management/manually-scaling-machineset.html#machineset-delete-policy_manually-scaling-machineset

Currently it is not possible to set this with a MachinePool.

This would allow a customer to make a change to the MachinePool and with deletionPolicy: Oldest`, you could scale e.g. 3=>6=>3 and MAPI would remove the three old/stale machines on scaledown.