from what I gather, if you have a 3G unlimited plan right now and next year you want a LTE handset, you can keep your unlimited plan. why? because at least for now - the same data plan applies to 3G and 4G devices. it's just called "unlimited data and email for smartphones". I went through the steps to upgrade my incredible to a charge - on the page where you select your plan, there is no distinction of a unlimited 4G plan. On the order summary page, there are some "features" that they add and subtract, but the $30 they charge you for is the same plan as before.
I am no authority on this however, it's just my interpretation (and I hope it's the correct one) because I wasn't planning on upgrading my handset until next year and I do want unlimited data once i'm on 4G.
(see attachment)