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First unless you shelled out the full price of the phone you did not agree to change from unlimited data. You lost it buy paying 1/4 the price of the device which can run as high as $800+
Additionally when you purchased the new phone the data plan changes to either pro rata for the data plan taking what you already used into the equation and then you would pay for the overages.
There is a way to make the data change to the next billing cycle but it was not offered to you at time of plan change.
There is also payments in advance and a $30 upgrade fee which is added to your account. This will make your next invoice much higher.
Good Luck