I think the following will happen: 20% of Blockland users will try it just to see what it is like. Of that 20% half will return to Blockland (lol a pun) right away. The other half will get full accounts and use it for a week, Of that half 80% will realize that it is an over censored (and really, what online game designed for children is not over censored) feature lacking, un-modable, suckish, piece of stuff of a game it is and come back to Blockland after weeks. The remaining people will never return (I remind you that this is 2% of the overall population of Blockland) and these people are 5 year olds tat could barely event a light switch.
But I also believe that our stream of new users will actual increase. This is because the Lego fans that recognize Lego Universe for the same reasons as above will start looking for alternatives.