I've never done the upgrade process through Apple.com, but I know on other websites like Amazon.com and BestBuy.com they don't give you the option to keep your unlimited data when upgrading. If anything they force you to pick a new data plan that the carrier currently offers when ordering the phone online, which stinks 
Does Apple.com let you keep your unlimited data plan when ordering online through their website?
Does Apple.com let you keep your unlimited data plan when ordering online through their website?