Here is the official Hertz policy:

Available at all participating locations in the U.S., Canada and select European locations. Maximum upgrade is to a full sized class vehicle with a minimum reserved compact vehicle.

First off, a CWAR (Compact Wagon) is not an "official" upgrade from a CDAR (Compact car). One is a sedan and the other is a wagon. It's apples and oranges. A CDAR is technically eligible for an IDAR (but of course you could get something else).

Second, even if you reserved a CWAR you really aren't eligible for say an IWAR. The typical upgrade path is for sedans only. Anything else (wagon, van, suv, etc.) is not eligible for the Gold upgrade. That's just the way Hertz's program works.

Now that said, the agents have a lot of leeway, so if you reserved a CDAR and asked nicely for something other than an IDAR, they can certainly give it to you, but they are under no obligation to do so, and even then, only on an availability basis, and at locations that participate.