Sorry ernestnywang, it's a black humour and I thought it's widely shared in FT but maybe only in other airlines' forum if you haven't seen this - I certainly didn't intend to confuse you.

SLC = Self Loading Cargo (a.k.a passengers) - I know it's very pessimistic but that sometimes describes the airline's attitude to pax especially when you complain.

Good point - I thought of F to J involuntary downgrade case but this is based on a fact that there is no upper class for F to upgrade to... when aircraft changes CX has to downgrade F SLCs to J cos that's the only best available product they could offer.

Y+ is different... it can be downgraded to Y, it also can be upgraded to J if CX decides to do so... that's where my question comes from... CX is different from QF - I haven't heard of QF sold a Y+ ticket but unable to deliver the product...only CX starts to sell Y+ tickets when only one or a few of their aircraft carry this product and can't guarantee its delivery.