My WBC op-up (and this was really last-minute...seat assignment discrepancy, and 1 empty seat up front) ended up posting as Y, in the fare class I purchased (Q/K, something like that).

So, no WBC credit.

Had an op-up with Continental, and same story. Flight posted as flown J, but fare paid was the original.

Of course, I've had countless domestic rebooks that went full-fare Y or First Class, so your mileage may vary...

Enjoy the cash and service.