I'm putting together an AONEx trip and doing SYD-AKL. I'd like to do the QF321 (LA metal) and there seems to be availability when needed. Now, if I change flights after ticketting in A to a flight with only D (ie anything other than QF321), will this mean a forced re-issue because of class, or will I just get reservations in D class since A is unavailable?
You don't need reissue.
Then, you can confirm 'D' inventry and wait 'A' inventory.