Can someone explain Cunard 'waitlist' practices?

[Essiesmom]

Last week I decided to upgrade our reservations on QE2 from M6 guarantee to Caronia. I decided to go no lower than C4. When I sat down with my TA and we went into the computer, it said all C4 cabins were waitlisted. No problem, we just went up to C3 and found a nice cabin in a good location on four deck. Not many of those left, either. Less than a dozen, I think. Anyway, I have been wondering what happens to all those people who are waitlisted if there are no C4 cabins available? Back in the dark ages when I sailed Home Lines, I had the feeling that a week before sailing they just plugged all those people into whatever was still unsold. And I think Carnival may be doing that with 'guarantee' bookings. There is so much Cunard expertise on this forum that I thought I would ask for educated speculation as to how they handle this. Are some of these people book in alower category with a desire to move up? Or are these people booked with a C4 guarantee? EM