They don't change your plan at any point as long as you're mailing less than 2500 people a month.
Really good deal at the moment (they changed their plans around and gave free users more content, because they make all their real money off companies sending out 20,000+ emails a month!)
(Not saying that there's no chance that the OP will out grow the free usage account, but you'd have to be building your list like nobodies business lol)
edit ====> one thing worth pointing out is that Mail chimp operates on a double opt in system, meaning that subscribers are meant to sign up, and then confirm that they wish to recieve your emails, can't remember what percentage it was, but we've had our account locked from FAR less complaints than you would have imagined to be possible! but a few friendly emails latter and we were back up and running again