I'm inclined to agree on the freemium thing. I may have an issue though when it comes to getting existing users on the free system to move over to a paid model. Do I grandfather existing users or force them to move?
That said, having people as free users and then cutting them off without warning could have a negative PR effect.
I think the easiest course of action is to put some wording in a prominent place on the site to the effect of "this is a free beta, but we will be moving to a paid model when we feel we are providing enough value". Then when the time comes, it won't come as a surprise. Then you put them on the 30/60/X day free trial (which you have anyway for new paid users) and they can transition.
Sure, you will lose some of those free users along the way, but unless they are providing value to you, free users are just going to be a drain on your limited resources.