I had a similar situation before:

"A" class on CX did not have availability on the flight I wanted at the time of booking.

I asked to be "confirmed" in "D", AND "waitlisted" in "A" at the same time.

Since I was confirmed in "D", my ticket had to be issued with "D" printed on it under the "class" field and "AAPRTW" (this was a different kind of RTW fare offered by CX) under "fare basis".

My "A" class seat cleared the day before travel, and when I got to the airport, the CX agent noticed that it "cleared" and gave me a First Class seat because she noticed I was on a "first class ticket". I had to remind the agent to "cancel" my "D" class segment which I won't be using anymore (or elese the rest of the flights would automatically be cancelled).