I believe you can upgrade at an Apple Retail Store and pay full price, even if you're eligible for an upgrade, thereby keeping your unlimited data.
However, at least for Verizon, the Apple Online Store won't let you pay the full unsubsidized price if you have an upgrade available on the line. Further, it forces you to select a new, capped data plan. This stinks, because it means there's no way to reserve an iPhone for store pickup. You either have to just show up and hope for the best, or order online from Verizon.