derik72090 wrote: Third, you said there's a way to make the data change to the next billing cycle, yet it wasn't offered to me at the time of the plan change. So basically they "forgot" and now I'm going to be paying for overages that I shouldn't have to pay for to begin with?

This is simply because you must still be eligible for your "current" data plan to make the change at the beginning of the next billing cycle.

When purchasing a new phone at a discount, you automatically lose unlimited data because you have entered into a new contract. You are no longer eligible for unlimited data and therefore could not have made the change effective at the beginning of your next billing cycle. Your 2 choices would have been at the beginning of your "current" billing cycle OR on the date of the purchase.