I'm not sure your TA is correct when he/she said there has to be a cabin available to take advantage of the sale. I believe @ArtistsWife had her booking repriced and her cabin category was waitlisted. If you reprice, instead of rebook, you retain the same cabin, booking number and original booking date. For rebooking, instead of repricing, you effectively cancel you current booking and rebook at the new price, which can't be done if all the cabins are waitlisted.