I just booked a TPAC flight on Qantas and paid to select my seats, but it says they're not guaranteed. Just wondering if that's legalese speak to cover them for extreme cases where they have to change my seat (change of equipment, etc), or is there a real possibility that my seats will not be guaranteed? Has anyone ever had a prepaid seat request changed on them?

I'm not too terribly picky about my seats, I just don't want to sit in a middle seat, and thought paying $50AUD to prevent that on 2 15 hour flights was worth it.