These "grandfathered" plans are still to this day one of the biggest reasons many people choose to stay on the carrier they are on as oppose to switching to a new one where an unlimited data plan is not possible to acquire.
Verizon is one of these carriers that has migrated to tiered data plans and will be making another switch this summer in their data plans to what they refer to as "data-sharing" plans. Along with this move, Verizon CFO Fran Shammo stated at the 40th J.P. Morgan Technology, Media and Telecom conference that "everyone [including those on grandfathered unlimited data plans] will be on data share."
This definitely comes as big news to many that absolutely despise the idea of not having an unlimited data plan. Verizon is using the change over to LTE as the main reason for this change in policy and to be quiet honest, it sort of makes sense.
Thankfully for us BlackBerry users (unless you tether), this really shouldn't present much of a problem. BlackBerry devices are known to be extremely efficient when it comes to data management so odds are, you probably wont even notice the change.
Your move, AT&T.