CX will not overbook that much. Most likely it will only overbook by about 5-10%. So it'll be very unlikely that CX will overbook a 346 capacity with a 744. Then why doesn't it sell it as a 744?

That's what yield management is for. They will determine just how many seats need to be jostled around.

One of the best airlines for yield management has got to be SQ. No matter how full the flight is, they still manage to put everyone in Y if their ticket is Y. Hardly any upgrades ever occur there!