how can they change it without our approval etc?
I guess I don't understand your statement. They CAN'T change it without your approval. If you continue to use it and pay the increase, then you DO approve the increase and will be billed at that rate. If you move on to another Verizon plan without unlimited data or another provider, then you DON'T approve the increase and won't have to pay it.
Your continuation with unlimited data is your approval. Changing from unlimited data to something else, either with Verizon or another provider is tantamount to not approving.