Hertz - Changed Gold rewards reservation to higher class, can't access it anymore

I'm a new Gold member and used 600 of the 800 points for a ICAR reservation for a one day rental in DCA. When I checked the reservation I saw that I could change my reservation to any car class, so I changed it to the Mustang Convertible. I got a confirmation mail and the points balance changed, so everything seemed fine.

Now some weeks later I wanted to change the return time and noticed that I can't access the reservation, but get the following error: "Promotional Coupon - The coupon is not available for this date and car. Please modify your request or remove the promotion."
The reservation is still listed, but I can't view the details, so I can't modify or cancel it on their web site.

Is it possible, that Hertz refuses to give me the car or shouldn't I worry? I don't care, if they won't give me the Mustang, but I would be nice, so I don't want to call them unnecessarily.

I'm a new Gold member and used 600 of the 800 points for a ICAR reservation for a one day rental in DCA. When I checked the reservation I saw that I could change my reservation to any car class, so I changed it to the Mustang Convertible. I got a confirmation mail and the points balance changed, so everything seemed fine.

Now some weeks later I wanted to change the return time and noticed that I can't access the reservation, but get the following error: "Promotional Coupon - The coupon is not available for this date and car. Please modify your request or remove the promotion."
The reservation is still listed, but I can't view the details, so I can't modify or cancel it on their web site.

Is it possible, that Hertz refuses to give me the car or shouldn't I worry? I don't care, if they won't give me the Mustang, but I would be nice, so I don't want to call them unnecessarily.

In short, I don't think you should have been able to reserve that class of vehicle with the base free-day points (convertibles are specialty class). My guess is the system was corrected or the system caught it after the re-confirmed reservation.

If I were you I'd call and make sure you still have a reservation. Not worth getting there and finding out you have no car.