Seems like there are only middle seats left for pre-assign on that flight (8B, 9BE, 16B, 20E). So I would say arrive at the airport early and see whether you can grab the exit row (if it's still available). You should keep calling UA once in a while and see whether they'll open it up for you. As far as I know, if you don't have any staus, they'll tell you that seat is taken. You must have gotten lucky when you got the exit row the first time.

Do you have any status yet?