The best way to find out for sure is to just call CX reservations and ask to use your seat guarantee on the lowest class permissible. You don't have to follow through with the booking if you just want to know the fare bucket and cost.

I've had MPO agents tell me the wrong thing a few times so its not much use talking to them about it particularly as they cannot book the flight for you.

If you're booking particularly last minute for sold out dates then checking on the website will just give you full fare Y tickets..