In most of my cases, the DONE down to Y has been because there was no D inventory available but J was. Therefore, the flight was confirmed in Y and waitlisted (or more likely the ticketing agent forgot to bother with this bit) D.

I never had a single J trip from LAX to AKL or SYD, always Y. Often the ex LHR transatlantic to IAD was Y as well (OK, I could get an exit row and it was a day flight for my body). Short hauls were usually OK.

The worst was a confirmed Y, waitlist D EZE to SCL. The inbound from SYD to EZE was delayed by 4 or so hours. I got AKL QF lounge to rebook my AONEx in D for the next flight. The gate agent then ripped up my J boarding card as the coupon only had confirmed Y for the original flight. That was a walk of shame.

I once got a Y to F upgrade on IAD to LHR. That was more to do with my occupation and BA Gold appearing on the pax print out for the CSD than anything else. I had already been upgraded Y to J because I offered to get off the overbooked aircraft. Then the right number of pax turned up but too many Ys.

I guess my paid D travelled Y long haul outstrips my paid Y travelled J by about 3 to one. As for AONEx, well I have got smarter, I have experience and I have asked others what to do. I have joined FT and I have a 1W ticketing agent as a girlfriend. I would suggest that bitter experience and the last two are the best mechanisms!

There are the links around here to check on the D availability for you chosen flights and you must remember to get your "entitlement" for D entered into the booking. That way, they will not op-up someone from paid Y ahead of you. You have paid, they have not. The entitlement also ensures that if a J seat is free and you are checked-in confirmed Y then the BP should spit out at the gate.