Originally Posted by ZSTWHK

For 1, I have OW benefits through Qatar but 0 miles would be credited due to ticket fare. I would have free seat assignment due to my OW tier so I bought the cheapest Cathay Y ticket. I switched FFP online from QR to CX after I made my booking and I had a notice that said that I need to call CX or I'd lose my seat assignment. After calling CX, they said that they would keep my CX (Green so no benefit) FFP on my booking but they confirmed my seat assignment made using my QR FFP.

When I got to the airport I had to double check if I'd still get my OW benefits because I don't trust anything that happens at CX these days. I explained to the agent at the check in counter again, that the booking has my CX FFP but I also have OW benefits through QR and showed them my electronic QR card. They understood and "made a note on my booking" that I'd credit to Cathay but use Qatar benefits. The rest worked out fine from there.

TLDR, just call, explain, be polite, explain again, remain calm