Originally Posted by
jetfan
I hear they are given on flights where the J-cabin has relatively many open seats, irrespective of the seat count in the Y-cabin.
And apparently they are offered on a random basis primarily to those doing an early web check-in -> this is last bit is confirmed by some posts here stating an early security number.
It would appear that it is also on the
website now (not in the exact wording):
As of now, you can also arrange an upgrade yourself at a self-service machine at the airport! If World Business Class seats are still available, buying an upgrade is a matter of seconds.
We may even offer you an upgrade at a great discount when you check in at a self-service machine, or online at KLM.com!
Gajan