Yes, I've definitely experienced rates that are NOT GUARANTEED, and it does explicitly say so in the email confirmation (which I didn't notice until after the rate had changed when I was viewing the reservation). But if the email says the rate is guaranteed, you should be fine, unless of course they try to accuse you of forging/modifying the confirmation somehow...

FWIW, I had this happen at a hotel once when on a corporate rate. The rez was booked around the end of the year for a February stay, and the negotiated rates for the new year were higher, and I didn't find out until check-in.