As I learned from rcowen the other day this is called yield management and it is propriatory information of the airline. Even for those of us who have been around for awhile it is a guessing game. There are so many threads here about award availability it makes my head feel like it is about to explode!
You can phone wp and inquire if there are seats available for upgrade on the flight you have already purchased tix for or you can check online at WP Award Travel Reservations prior to buying your tix.
There are plenty who make it a habit to get as much information about their frequently used routes as to best optimize their opportunity for upgrade. (eg: as a gold I have never been upgraded on a red-eye out of sfo - hence I began avoiding those flights even if more economical or convenient) There are a number of posters here who are both op & wp (elite in both in come cases) My understanding is that they have a great amount of flexibility on which account the miles go into and probably (I would) make the decision based on who has the best bonuses going on at the time. Whew! You are asking this in FlyerTalk! The short answer is on these boards yes and the reasons are many and varied. price-elite benefits-mileage earning opportunities-service-equipment-food-entertainment-etc.-etc.-etc.