As you already know Verizon has eliminated their unlimited data plan, and all new lines and upgrades will require you to pick from one of their new data plans. With the Samsung Galaxy S III pre-orders beginning to arrive in the hands of lucky owners, there are a few reports of activation of the device causing their unlimited data plans to wrongfully be removed from their plan. If you placed your pre-order of the device before June 28, the start date of the new Share Everything plans, you should have been locked into your unlimited data plan again. If you placed your order on or after June 28, unfortunately you are stuck with the new plans, and nothing can be done at this point.
If you notice that your plan has wrongfully changed, don't panic. Simply dial up Verizon (*611) and explain to them the error and they will change things back to the way they should be for you. Be sure to let us know in the forums if you had to have your plan corrected, or if activation went smoothly for you.
Source: Android Central Forums