If on a single ticket CX-CX, you will be protected.
If there is any business seat left, you will get it. Fare bucket would not matter (except for a possible priority list if many pax are delayed and few seats available).

I don't know what happens to status points if you are downgraded, but I am afraid that you will be credited for points in the class you travel and compensated monetary.
Of course, you can also choose to wait in HKG for the next flight where J seats are available.
There are like 10 flights to TPE and 8 to PVG, so pad some time in your itinerary to reduce the risks if you wish to.