What chance? In the last 6 years I have been upgraded at the gate on an international flight only once. The next closest shave was a day before departure. There are a couple of instances where I recall being upgraded 3-4 days before departure. The rest of the time, at booking or weeks before departure. This is based on a sample of perhaps 60 flights (all long-haul international), covering every international destination that AA flies to.

And as an EXP, when all else fails, I've never been turned down on an appeal to revenue management to release a seat for me (assuming there are revenue seats available, of course). When was the last time that any DL flyer was able to get RM to make an exception?