I asked this same question recently. They still aren't completely sure what they'll do when the game is done, but will most likely just stop updating the free version at some point nearer to completion.
what will happen with the free version when the game is finished? will we be able to play the full game for free or will it be turned into a demo/trial version at some point? sorry if this info is posted somewhere, but i haven't been able to find it.