same thing happened to us in dec/jan....we actually had 2 1-week reservations made months in advance on hertz.com with very good pricing due to corporate discount...a couple of weeks before arrival received new confirmation from SXM office, but with much higher rates (almost 3x on one, 2x on the other)

contacted SXM office and was given the same "originally booked car class not available" excuse, but they gave us a courtesy discount to make the rate slightly more palatable...wound up keeping the first rental period, but used another company for second