I'm not beating up on QF - and I know this is on a DJ thread but the discussion has well and truly turned into a comparison, so will add my refutation of changes of preallocated seat not happening on QF and not happening for WPs.
Have been moved on a recent J long haul International from 1A to a middle section, after my preallocated seat having been showing in my booking for many weeks before departure and the change being notified only in the Lounge, when I enquired. Had not noticed at chec kin, as it was the first of a number of legs and the check in person was woefully inadequate and took forever to complete the formalities. All I wanted to do was escape his clutches.
My fault, you could say, for selecting 1A, with the possibility of having to move for someone needing a bassinet. Yes, I know the seats are not guaranteed, but why do those travelling with a baby get to choose on Departure Day if they need a specific seat. And what happens in these situations if there is no other J seat available for reallocation? There was no F on the flight.