In the MachineConfig definition, there are many Ignition fields that cannot be modified day-2 since the MCO cannot make those updates to existing nodes.

But sometimes, the user would like new nodes to come up with a new configuration, and it is not doable today without modifying either the pointer ignition or the MCS served ignition before giving it to a new node.

For example, let's say I installed OCP 4.10 with a butane config to modify disk partitions. Sometime in the future (e.g. 4.14) I have new hardware needs and want to add new disks with a different partition scheme. The existing scheme in the MachineConfig cannot be removed without the MCDs complaining for existing nodes (the controller will correctly render the new config, but no nodes can perform the update). A workaround today is to ask the MCS for ignition, modify the ignition by hand, and then add that to the new node, but that's not ideal (and not very feasible outside of non-cloud deployments)

Thus we should have a knob, probably within the global MachineConfiguration option, to override this. A user can turn on a flag to allow changes to e.g. disk partitions. The existing nodes will just perform a no-op update, and the new nodes will get the new config correctly.