I'm a new plat and didnt get an upgrade in CDG a couple of weeks ago. Given some BS about that it was subject to availability up to 6 days prior to rental - load of crap. But the agent was happy to upsell! I booked a E-series, was offered a 420 (suitcases didn't fit that car) and was then made to wait 45min to get the E-series. While all along there was a 525 in the lot "...oh, but that is a different class... would you like to pay and upgrade to that?".
Solution: Complain to Sixt. Got a EUR100 voucher. Yet I doubt I'll book with Sixt again. Accorhotels just started their partnership with Hertz so I will give that a try.