Upgrading to 4s from a Blackberry - Don't want to lose data plan

Hi all,

So I've seen (even in the stickies in this very forum) that it's possible for me to upgrade from my Blackberry to an iPhone, keeping my unlimited data plan. 

However, when I went to pre-order the 4s this morning, I only had choices of one of the tiered data plans, and saw no unlimited option at all, so I didn't go through with the pre order.

Is this something I should be able to do online? Or would I have to call or go into a store to get this accomplished.

Thanks for your time!