I am Diamond by sixt.
Rented a mazda 3 both times for 32 Dollars a day (incl. all insurances)
Rental two weeks ago: upgrade to Ford Edge
rental now:prepared a Buick asked for a upgrade to a 7 seater (V1), gave it to me without a problem (on a week rental)
Have to say, they really respect the Diamond card and give me any car from the Lot, that I want. The only cars they are not allow to give out on Upgradesare Audi A6 and Mercedes.