There have been many reports concerning Airbus A340-600s... but I am sure that the problem is being resolved. I have full confidence about these beautiful airbuses. I am flying the A346 this upcoming Wednesday.
Seeing the Toronto flights are now rerouted through Vancouver. If there is an equipment problem with CX 838 or 839, passengers are most likely be placed on the two other YVR-HKG flights. If it is a full load, CX will most likely substitute with an Airbus A340-300. But it is hard to predict.
Don't worry much! You will be fine and most likely you will get to fly the brand new Airbus A340-600.
Carfield