If people are always getting stuck like this, why book the codeshare
I always get my TA to do everything on one ticket but always use flight numbers from the operating carrier, not a codeshare. So my ANZ flight might be AC9999 but I will always book it as NZ105 so I can control which booking class things go into.
Seems like it would be a great way to avoid all of this...unless there are some serious savings involved