Regarding the OP's question, I know most people do online check-in now, with most having boarding passes emailed or are using the Qantas App, but in the case where people do online check-in the old-fashioned way and print a static physical paper boarding pass, how can Qantas move people? I assume they can page people at the terminal and/or page people in the lounge or force them to recheckin if they are checking in luggage, but if you are doing HLO domestic and you have a paper boarding pass I would be ignoring those personal pages of the PA - unless the entire flight was delayed or an equipment swap, then the only reason for this personal page is to give you a worse seat that what you have already selected right?
Would be interesting to see if its the bassinet rows being required, and/or broken seats and unservicable IFE as the main reasons for these seating changes post online chekin?