The 12h upgrades are locked out before the 12h period - I tried to upgrade a 7PM WAS-NYP when I was checking in for my 7AM NYP-WAS (around 6:40AM), and the lounge agent tried and said that the computer wouldn't accept the request until the 12h window opens. Don't know about the 48h upgrades, but there seem to be not too many of those around...