I did a point run between TPE-SGN. I think the practice would be to put you on the next available flight (if it is on one ticket).

But there are alternative ways in which CX deal with this. My TPE-HKG flight was delayed on the day and I was offered an alternative flight with another airline (though I think would be a rare occurrence). I made clear that I am doing it only for the points so I asked them to confirm whether I will be credited for whatever that I paid for. These points had since been credited to my account so I think even if they "downgraded" you, you would still be entitled to the points and miles that your original ticket would give.

You probably just need a confirmation from the staff handling this so you get some kind of bare protection if you need to make a claim officially.