Well, if you look at costs, the answer is pretty obvious.
Power Pass....$239 Preferred Pass.....$344
One day park to park ticket ... $155
So if you are planning on visiting more than two single days, the Preferred Pass is still cheaper. If your visits will not fall into the blockout dates for the Power Pass, then the Power Pass pays for itself in less than 2 visits.
You can upgrade at Guest Services inside or outside the park. You have to update before you ticket expires. So if you have a 2 day ticket, you need to upgrade on the second day you use the ticket.