You can just keep checking. I don't know if there is a time that certain seats (i.e. exit rows) can be booked by non-elites, but on the day of travel, many elites will change their flights. And you can just go to the gate and say "I'm in a middle, do you have any better seats?" If they have a better one, they'll give it to you. Usually even exit rows and bulkheads.