FWIW, I've regularly upgrade from WTP to CW LHR/ORD (both directions) and so long as I booked 4-5 months in advance, it's never been a problem at all. As of the last six weeks I've been looking to book a few LHR/ORD trips for 2013, one as early as June, and have found nothing. Even December is slim pickings and early next January is blank (except for one day midweek/month).
DFW is the same, as is MIA. Anyone have any ideas why this would suddenly change?