First I've heard of this. I've never heard of anyone upgrading through Amazon and retaining unlimited data. They force you to select an existing data plan.
EDIT: Just verified through attempting to use an available AT&T upgrade. There is no way via the online purchase to retain a grandfathered unlimited data plan. You could always take the new data plan and then call the carrier after the fact, but there is no guarantee that they will give it back, especially when they didn't get the sale.
EDIT 2: Some more info on this;
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So to reiterate, if you upgrade through Amazon rather than AT&T/Verizon, you will initially lose your unlimited data plan. You can then call customer service at the carrier and request a ticket be opened to restore your unlimited data plan (Verizon gives you 60 days to do this, AT&T I'm unsure of). However, given that you sent your money to someone else instead of them, and they have a new 2-year contract with TWO early termination fees (carrier for 24 months and vendor for 6 months), they have no motivation to restore the unlimited data plan. So, do this at your own risk.