Not True, I spoke to AT&T twice about switching from an iPhone to a Blackberry before my company allowed iPhones for Work, and they said if I switch from an iPhone Unlimited to a BB, I lose the Unlimited Data, the only way you keep the Unlimited Data is by moving from whatever you were on at the time, to the same MFG model, so iPhone to iPhone and BB to BB, I was told this twice and so was the guy who handles the phones in our office.
Eventually they will phase out these Granfathered unlimited plans, they all want to go to a better cost model.
I just can not wait to hear what Verizon winds up doing however I do not think they will have nearly as many iPhones as AT&T does, but I am sure they will restrict data and make you pay, it is the VZ way.