Just a thought...I'd guess that the rate to ensure that you get a convertible would be much higher than the rate you'd pay for the station wagon, so you'll likely be unnecessarily paying extra for the station wagon. Thus, why not just make 2 reservations?

If you really do want to keep it under one reservation and do a switch, I've found that just showing up and making an excuse that you'd like to change cars because of discomfort or something is normally accepted without much trouble. I've done this in many places, including Germany (where they speak English well, so no problem with not speaking German).

Switching cars is most-easily done at a busy airport location, where they have plenty of cars available. I have run into more difficulty with trying to switch cars in small locations where they don't have a lot of selection.

Have a nice trip.