You can buy it for an individual segment AFAIK. The cost from DCA-PHX is $150, but the chance of it being available is next to nil because of the high density of elites on this route. I would call 30 hours prior to departure and keep checking by OLCI leading up to departure as the upgrade may be offered if there are any cancellations up front or the off-chance the flight isn't fully booked up front.