If you accept a substitute vehicle then you probably won't be able to argue for the points back. If you don't pick up at all, you can try calling Hertz to make the case they should refund you the point, but it may require a bit of patience and teeth pulling. A better approach is to just call the location about 48 hours beforehand to confirm they have a convertible on-hand and can hold it for you, or get one there in time for your reservation pickup time. Explain that you're going a points booking that can't be cancelled so you want to make sure that the vehicle you reserved is available.