Ok.. this will be a hot topic I see...

I really don't know what CX will say.. so like I said, let OP write in and see.

If it happened to me and I was op-up'd, I wouldn't dare make too much of a big deal.. because I would be embarrassed in the first place as the ISM would know that I was op'up'd from J. If the flight is really full, and I needed the sleep , I would try asking the ISM if they could switch seats with me - whether in J or even Y... well, ok, not Y as it would be better to sit upright in a F class seat than a Y class seat.

I wouldn't dare dispute the cc charge because it is not a valid complaint. I would however, tell CX that I didn't get my J class seat because I paid for a flat seat and I didn't get one.

But let's just say theoretically, that on the day of your flight, that aircraft snagged, and CX decided to put you on another aircraft with no flat seats. What then? CX fulfilled their obligation and contract in taking you from A to B.. though of course having a flat seat is one of the perks of flying CX, but flat seat or not, the fare is exactly the same.

We don't know exactly what CX will say or do... everything here is speculation. Now where are the "CX employees" when I need them here ?

Just write in and see... we may all be very surprised.