Forgot mention an important factor if you are planning on travelling during busy season and expect to make date changes. The inventory in all classes is tight during peak travel or on convenient flts. It is usually much better than the inventory for award travel (even with elite status) but still far worse than corresponding F, J or Y farecodes. So, if you want to be impulsive about dates travel during off-peak season. (M class in coach is particularly tough)

Most of the times they release some decent inventory a week to a month before a flt on international travel and as it gets closer to the travel date, they release the inventory for award travel and upgrades so it vanishes.