I'm on QF25 shortly; I changed the day of travel due to convenience and other factors. I originally had a prime 2 class 744 seat. When I changed the new one was still a 2 class 744, but all good seating, including exit rows were taken.
I Noticed on the weekend that a 743 had been substituted and rows 20-25 were completely free... not for long...
Funnily enough, I might not have changed my flight day if it had originally been a 743.