The way I had it explained to me at the Verizon store is that if you have a 3G phone with unlimited data, and later plan on switching to a 4g phone after the tiered rates go into effect you will need to get a new plan and will lose unlimited data. However if you have an unlimited plan on a 4g phone, and you change to a new 4g phone after the tiered rates have gone into effect and renew your contract you can keep your same plan and unlimited data. They say you will be grandfathered into it.
This is the reason I went for a 4g phone now to ensure that I will be grandfathered in on my future 4g phone purchases and able to keep my unlimited data.
One thing to keep in mind though..the tiered data plans that they are rolling out may actually be able to save some people money. If a person is not using as much bandwith, it does not make sense for them to pay for an unlimited plan. For example I know my mom is paying for unlimited usage on her iphone 4, but I know she doesn't use much bandwith at all...I would be surprised if she used 1GB a month....It all really depends on the amounts that they will charge per GB of traffic. But for my own peace of mind I think it is always better to have an option to use as much as I want and not have to worry about it.